2—
High Tension

Were the million volts the only criterion—were size of current and steadiness of operation of little importance—the quest would have been over soon after it started. Several techniques for obtaining very high, fleeting voltages across discharge tubes succeeded by or before 1930. None gave rise to a useful beam of positive projectiles. Nonetheless they are worth attention, since some of the problems they raised, and workers they employed, recur in our story. In addition to these impulsive methods, two others for obtaining steady high potentials directly were under development in 1930. For some purposes they held an advantage over the cyclotron.

The Impulsive Way

Nature provides big voltages gratis to anyone bold enough to play with lightning. In the summers of 1927 and 1928, three members of the physics institute at the University of Berlin hung an antenna between two mountains in the Italian Alps 660 meters apart. In their definitive arrangement (fig. 2.1), a string of heavy-duty insulators, provided free by their German manufacturer, kept the antenna and the probe line attached to it from ground; the potential reached by the antenna was controlled and measured by the air space between a metal sphere dangling from the probe line and an earthed sphere hanging beneath it. During thunderstorms, the antenna rose to a very high potential over ground, sending sparks between the spheres across as much as 18 meters of pure Alpine air. The intrepid experimenters calculated that, in that case, they were dealing with 15 million volts. The two who survived the experiment, Arno Brasch and Fritz Lange, returned to Berlin to construct a tube that might stand up to a good fraction of this voltage.^[9]

In the comparative safety of the Allgemeine Elektrizitätsgesellschaft's research laboratory in Berlin, Brasch and Lange

[9] Brasch and Lange, Zs. f. Phys., 70 (1931), 10–1, 17–8. Their unfortunate companion, Curt Urban, was killed during the experiments of 1928; Livingston, Particle acc. , 2, misdates the experiment and Livingston and Blewett, Particle acc. , 22, kills the wrong man.

Fig. 2.1
Brasch and Lange's lightning catcher. E and H are the
spheres between which the discharge occurs; AE, the
antenna; a,a, insulators; b,b, conductors; d, a grounded
wire. Brasch and Lange,  Zs. f. Phys., 70  (1931), 17.

tested designs for a sturdy discharge tube with the help of an impulse generator able to reach 2.4 MV. Its principle, the "Marx circuit," may be clear from figure 2.2. A transformer at the center of the hexagon to the right of the diagram delivers rectified direct current via the indicated diodes to the string of n capacitors C , which charge in parallel each to the potential V . When V suffices to drive a spark across the gaps F , the capacitors connect briefly in series, the voltage on the last plate rises to nV , and a spark jumps to the top electrode of the constantly pumped special discharge tube figured on the left.^[10] Such generators served the electrical industry to test the characteristics of insulators and other equipment during flashovers. The grandest ever made, which could manage 6 MV, was built by General Electric in 1932.^[11]

Another industrial high-voltage instrument adapted to nuclear physics was Southern California Edison's million-volt cascade transformer at Caltech. This object did not satisfy Rutherford's requirement of convenient size: it filled a room 300 square feet in area and 50 feet high.^[12] As we know, it was adapted to the

[10] Brasch and Lange, Zs. f. Phys., 70 (1931), 29–30.

[11] Livingston and Blewett, ibid., 21.

[12] Lauritsen and Bennett, PR, 32 (1928), 850–1; Kargon in Shea, Hahn , 77; Ising, Kosmos, 11 (1933), 144–7, for industrial forerunners of Lauritsen's design.

Fig. 2.2
Brasch and Lange's discharge tube and impulse generator. Voltage from the
transformer Tr multiplied by the string of capacitors discharges across the
constantly pumped laminated tube. Brasch and Lange,  Zs. f. Phys., 70  (1931), 30.

production of x rays and ion beams by C.C. Lauritsen, who had been so inspired by a lecture by Millikan that he gave up making radios and enrolled at Caltech in 1926, at the age of thirty-four, as a doctoral student. He developed a very fine experimental technique, precise in measurement and simple in style. European visitors judged him to be one of them, free from "the technological extravaganzas that Americans like so much." Brasch, who had reason to know, rated Caltech's engineer turned physicist "an uncommonly good experimenter" with outsized electrical apparatus. Lauritsen's first big challenge as a superannuated graduate student at Caltech was similar to what Brasch and Lange faced at the same time: to make a tube that could stand up to a million-volt generator.^[13]

[13] Lauritsen, "Stipulation" to Patent Office, Sep 1933 (Lauritsen P, 1/8); Holbrouw in Weart and Phillips, History , 87; quotes from, resp., W. Elsasser to Joliot,13 Sep 1936 (JP, F28), and Brasch to Szilard, 27 Jan 1939 (Sz P, 29/306).

A third group—Lawrence's chum Tuve and his co-workers at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington—had come to a similar situation with still another approach to high potential. Tuve's Ph.D. thesis, completed at Johns Hopkins in 1926, was a measurement of the height of the ionosphere via radio pulses, using a method devised by Carnegie's theorist Gregory Breit. Tuve and Breit continued this work; but they also wanted something novel to mark Tuve's arrival, and decided to try to force the ordinary Tesla induction coil up to several million volts in the service of atomic and nuclear physics. The rationale offered the officers of the institution for this undertaking in the department's report for 1926/27 was that the Tesla coil might be driven to 30 MV to make cosmic rays in the laboratory. Further formal justification for the development of a program so obviously alien to the department's mission was found in the consideration that, as Tuve put it, "the problems of terrestrial magnetism will never reach a really satisfying solution until we possess an adequate understanding of the basic phenomena of magnetism itself." How else secure this understanding than by knocking the nucleus apart? On this flimsy pretext, the department allied itself with the U.S. Navy, which loaned it transformers, condensers, and a glass blower, and steamed forward to create one of the most important laboratories in the world for nuclear physics during the 1930s.^[14]

The Carnegie's coil in its definitive form consisted of a primary of a few turns of copper tubing wrapped into a flat spiral around a secondary of many thousand turns of fine insulated wire wound in a single layer on a pyrex tube (plate 2.2). The primary was driven by the discharge across a spark gap of a very big condenser the navy had used in an old wireless transmitter (fig. 2.3). If tuned to the frequency of the oscillating primary discharge, the secondary could acquire a greater peak potential than the air around it could sustain. By immersing it in oil under pressure, however, Breit and Tuve arranged that the secondary held all the primary delivered,

[14] Abelson, PT, 35:9 (1982), 90–1; CIW, Yb, 26 (1926/7), 169, and 31 (1931/2), 229; Tuve to Lawrence, 13 Nov 1926 (MAT, 4), remarks to the AAAS, 1931 (MAT, 8), and, JFI, 216 (1933), 1–2; J.A. Fleming to F.W. Loomis, 16 Dec 1931 (MAT, 8).

some 5.2 MV (they thought) with a secondary half a wavelength long. Although this estimate appears to have been too high, whatever they had would have driven an effective beam of alpha particles had they had any way to apply their coil. And so, like Lauritsen and Brasch and Lange, having solved the problem of high tension to their satisfaction, they set out, in 1928, to make a tube to withstand it.^[15]

Fig. 2.3
Schematic of the installation of the Carnegie Institution's 5=mV Tesla coil.
Breit, Tuve, and Dahl, PR, 35  (1930), 56.

Lauritsen fielded the first effective tube, which required a scaffolding fourteen feet high of good California redwood for its support (fig. 2.4). Its fundamental features were adapted from the high-potential x-ray tubes developed at General Electric by W.D. Coolidge, who reached some 350 kV in 1926 by shielding the glass walls with a copper tube (fig. 2.5). In this way he defeated the buildup of charge in the walls from electrons driven into them; and hence also the discharges to the walls that punctured the tubes and made the main obstacle to increasing the voltage of x rays. To go beyond 350 kV, Coolidge recommended cascading two or more tubes (fig. 2.6), passing the electron beam through thin windows that would act as anodes for one tube and cathodes for the

[15] CIW, Yb, 26 (1926/7), 169, and 27 (1927/8), 208; Breit and Tuve, Nature, 121 (7 Apr 1928), 535–6; Breit, Tuve, and Dahl, PR, 35 (1 Jan 1930), 53–7 (the definitive coil). Livingston and Blewett, Particle acc. , 17, doubt the 5.2 MV.

Fig. 2.4
Cal Tech's high-voltage x-ray installation. The corona
shields are attached to the metal rings at the joints
between the gas-pump cylinders constituting the
tube; the very long cathode reaches almost to the
tube's bottom. Lauritsen and Bennett,
PR, 32  (1928), 852.

next and stop positive ions that might otherwise gain high energy and make difficulty. Lauritsen's tube had four segments, to correspond with four cascaded transformers; each tube was twenty-eight inches long and twelve inches in diameter, and made, appropriately to its location, from the glass cylinders then used in pumps in gas stations. The segments joined at steel rings, which also supported the internal wall shields and, externally, circular slips of tin foil to protect against corona losses. The high-potential end of the cascade transformers fed the long central electrode of the tube (a three-inch steel pipe extending to within an inch of the earthed target) through a water resistor. The tube itself was continually pumped to retain a good vacuum. By August 1928 Lauritsen and Bennet could put 750 kV across the tube with no difficulty, and obtain x rays capable of penetrating over 2 cm of lead.^[16]

Fig. 2.5
Coolidge's first design for a high-voltage x-ray tube. Its chief feature is the
metal shields around the electrodes, k and t, which prevent buildup of charge
on the glass. Coolidge, JFI, 202  (1926), 696.

Lauritsen patented the design. General Electric had supported Coolidge's work in the hope that very penetrating x rays might be especially effective against cancer. A cheap and efficient high-tension plant had commercial as well as humanitarian possibilities, whence the considerable interest of both industry and medical philanthropy around 1930 in large numbers of volts. As Lauritsen wrote in his patent application that year, his tube, then able to operate at a million volts, could serve "as the full equivalent of

[16] Lauritsen and Bennett, PR, 32 (1928), 851–3, 857; Coolidge, JFI, 202 (1926), 695–6, 719–21.

radium in the treatment of disease, or for therapeutic purposes." A gram of radium then cost $60,000 to $70,000; an x-ray plant of moderate potential, about $30,000. Rewards could be high. And hopes. Many victims of cancer basked briefly in the million volt x rays of the Kellogg Radiation Laboratory.^[17]

Fig. 2.6
Coolidge's cascade. Two tubes of the type shown in fig. 2.7 are joined together
so that the anticathode of one becomes the anode of the other. Coolidge,
JFI, 202  (1926), 720.

Meanwhile Brasch and Lange were adapting Coolidge's design for use with the AEG impulse generator and the Alpine lightning factory. Late in 1929 they had a porcelain tube over seven feet long, with walls an inch thick studded with 300 nickel rings. They solved the problem of stray electrons not with complete shielding of the walls, as Lauritsen provided, but by enough rings of sufficient capacity to prevent large buildup of unwanted voltages. This great studded stick—made so long in order to space out the longitudinal potential drop—could stand a surge of 1.2 MV. Nine months later, in August 1930, they miniaturized to three feet by immersing the tube in oil to cut out corona discharges from the now high-potential external walls into the air. Careful study showed that the hurdle to going higher was a creep of electrons along the walls between the metal rings. They nipped the creep in a new tube composed of alternating rings of paper, aluminum, and

[17] Benedict Cassen to Lauritsen, 8 Oct 1932 (Lauritsen P), on cost of x-ray plants; Tuve, memo, 23 Jan 1932 (MAT, 8), re radium; supra, §1.2.

rubber of different widths. The spacing of the metal disks remained close, while the creeping distance between them increased enough to discourage the most persistent electron.^[18]

Although the vapor pressure of the rubber prevented them from achieving a very high vacuum, Brasch and Lange got spectacular results: the laminated heap withstood the maximum impulse of the generator, 2.4 MV, with transient currents of 1,000 amps; the zippy cathode rays thus produced made x rays that could penetrate 10 cm of lead, and offered a new medical possibility in themselves. Rather than apply very hard x rays against deep cancers, why not try short bursts of million-volt cathode rays, which deliver most of their energy as they come to rest? "Then the irradiation can work very effectively deep within the body without doing so much damage to the parts near the surface." Brasch and Lange's main goal, however, was to adapt their tube to the acceleration of positive ions. They managed to obtain a stream of hydrogen ions at 900 kV, which they thought too low to provoke the transformation of elements. They returned to the Alps to continue their alchemy.^[19]

The Department of Terrestrial Magnetism was most impressed by "the spectacular performance" of Brasch and Lange's laminated tube. "They are to be congratulated without reserve," wrote Tuve and his associates, whose own handiwork did not perform quite so well. They had stayed close to Coolidge's design, multiplying segments and decreasing size by immersing the whole in oil. By 1929 their Tesla coil was driving a tube with six segments at 850 kV and one with fifteen segments (all seven feet of it) at 1.4 MV; later, by heatworking the glass to remove bubbles, they operated a twelve-segment tube about a yard long at what they thought was 1.9 MV.^[20] Just before Tuve's group learned about the spectacular performance in Berlin, they succeeded in detecting cathode rays and x rays from the tube and fixing their energies at

[18] Brasch and Lange, Nwn, 18 (1930), 16, 765–6, and Zs. f. Phys., 70 (1931), 21–9. The laminations are suggested in fig. 2.3.

[19] Brasch and Lange, Zs. f. Phys., 70 (1931), 30, 33, 35, and Nwn, 21 (1933), 82–3; Badash et al., APS, Proc., 130 (1986), 202.

[20] Tuve, Hafstad, and Dahl, PR, 36 (1 Oct 1930), 1262 (quote); CIW, Yb, 27 (1927/8), 209, and 28 (1928/9), 214; Tuve, Breit, and Hafstad, PR, 35 (1 Jan 1930), 66–7; Tuve, Hafstad, and Dahl, PR, 35 (1 June 1930), 1406–7.

1,250 to 1,500 kV. It remained to accelerate protons, and also to develop a current in the tube sufficiently strong to permit measurement of the intensity, as well as detection of the existence, of the penetrating radiations. The Tesla coil as used in Washington, with its very brief duty cycle, was not a competitor for the medical purse.^[21]

In the report of their work for the year 1929/30, Tuve's group pointed out that the tube perfected with the aid of the Tesla coil already outperformed it; and they called for a new generator "in order to adapt this new tool effectively to the studies in atomic and nuclear physics for which it was developed."^[22] The tube won them a prize from the AAAS and some attention: page 1 in the New York Times and an advertisement in Time that with their x rays—reported as equivalent in gamma radiation to $187 million worth of radium—they might split atoms and cure cancer. Lawrence congratulated them on "such a fine recognition," meaning the prize, not the puff.^[23] Their disappointment with the performance of their Tesla coil evaporated in this sunshine. They tried to accelerate protons, succeeded on December 8, 1931, and directed the beam overambitiously to shattering the elements. Nothing detectable happened; so small was the beam that "further attempts seeking evidence of nuclear disintegration with this set up [were] discontinued." Lawrence again offered encouragement. The tube, he reminded Tuve, was "a terribly important thing!!!—as I have emphasized to you many times (and you try modestly to give Coolidge the credit for)." It remained to find a way to make the tube produce something other than a prize.^[24]

[21] Tuve, Hafstad, and Dahl, PR, 36 (1 Oct 1930), 1262; CIW, Yb, 29 (1929/30), 256–7, and 30 (1930/1), 291.

[22] CIW, Yb, 29 (1929/30), 257.

[23] New York Times , 4 Jan 1931, 1; Time, 17:1 (12 Jan 1931), 44; Tuve, Hafstad, and Dahl, PR, 37 (1931), 469, and statement of 19 Feb 1935, re the prize of 1931 (MAT, 14/"lab. letters"); Lawrence to Tuve, 4 Jan [1931] (MAT, 8), adding that Lauritsen had expressed "keen admiration for your work."

[24] CIW, Yb., 30 (1930/31), 292–3; ibid., 31 (1931/32), 231, 229 (last two quotes); Lawrence to Tuve, 15 June [1932] (MAT, 4); Tuve, Hafstad, and Dahl, PR, 39 (1932), 384–5. Cf. Tuve to Lauritsen, 30 Mar 1933 (Lauritsen P, 1/8), advising against trying a Tesla setup for disintegration experiments.

The Old-Fashioned Way

And one was provided. The inventor, Robert Jemison Van de Graaff, conceived a grand idea at the onset of his career, as a Rhodes scholar at Oxford, where he arrived with an engineering degree from the University of Alabama and professional experience with the Alabama Power Company. From Oxford, where he earned his Ph.D. in physics in 1928, he went to Princeton as a National Research Fellow, and soon had a prototype accelerator working at 80 kV. Its principle would have been plain to Benjamin Franklin. An endless belt of a good insulating material running vertically between two pulleys picks up electricity from a point discharge at the bottom and delivers it, again by point discharge, to a large insulated spherical conductor at the top (fig. 2.7). The lower electrical spray comes from any rectified source. The upper spray continues, irrespective of the potential attained by the sphere, which exerts no electrostatic force at its internal surface, until the field at the external surface suffices to break down the air. In a later refinement (fig. 2.8), a second set of points adroitly placed removed electricity of the unwanted sign from the sphere on the belt's downward journey and eliminated the rectified source by connections that allowed the amplification of any slight charge present on the belt. Since the practical limit to the potential of the sphere is the dielectric strength of the air, the way to millions of volts was to increase the sphere's radius (and so lessen its external field at a given potential) and the dielectric constant of the surrounding medium (and so raise the field at which breakdown occurs). To make good on the second possibility, the entire machine must be encased in a vessel that can be evacuated or filled with a gas or fluid under pressure. The first small pressurized model operated in 1932.^[25]

By mid August 1931 Van de Graaff could charge a brass sphere mounted on a glass stick to about 750 kV. Between two such spheres, one positive and one negative, an inspiring potential difference of 1.5 MV could be maintained. It, and the trivial cost,

[25] E.A. Burrill, DSB, 13 , 569–70, and PT, 20:2 (1967), 49–50; H.A. Barton, D.W. Mueller, and L.C. van Atta, PR, 42 (1932), 901; Van de Graaff, Compton, and van Atta, PR, 43 (1933), 152–5.

Fig. 2.7
Principle of the Van de Graaff generator. Charge
sprayed on the endless silk belt at the bottom leaves
by corona discharge at the top; it is derived in the
first instance from a transformer. Van de Graaff,
Compton, and Van Atta, PR, 43  (1933), 152.

about $100 for the entire outfit, inspired Karl Compton, who brought Van de Graaff to MIT as a research associate (he became associate professor in 1934) and arranged some heady publicity. The newly formed American Institute of Physics held a dinner for scientists and journalists at the New York Athletic Club; the machine, in an alcove in the dining room, looked like "two identical rather large floor lamps of modernistic design." Van de Graaff demonstrated for his supper, and also for Paramount and Pathé news; he allowed that he saw no difficulty going to 10 MV with two balls each 20 feet in diameter on towers 20 feet tall. Compton misguessed that this big machine would provide alpha particles in a current "so enormously larger than that from radium, that the experiment opens up the possibility of transmutation of the elements on a commercial scale;" and he miscalculated that it could be done for a few hundred dollars.^[26]

[26] Quotes from New York Times , 6 Nov 1931, 1, 6, and 11 Nov 1931, 1, 17; J. Boyce to Lawrence, 17 Nov 1931 (3/8); Van de Graaff, PR, 38 (1931), 1919–20;Van de Graaff, Compton, and van Atta, PR, 43 (1933), 154.

Fig. 2.8
An improved Van de Graaff generator. The points are
arranged so that the belt charges the sphere when
going down as well as when going up; the system
works with any stray charge, no transformer being
required. Van de Graaff, Compton, and van Atta,
PR, 43  (1933), 153.

There remained what Lawrence, who recognized Van de Graaff as a competitor, called the "old problem of a high vacuum tube." Those who thought they had solved the problem regarded the matter differently. As one of Lauritsen's students wrote him after witnessing one of Van de Graaff's demonstrations, "His scheme is really very good and actually works . . . [and] would make a very fine combination with one of your tubes." Lauritsen eventually did build a Van de Graaff machine.^[27] Tuve's group rushed to do so. In September 1931 Tuve drove Van de Graaff and his easily portable equipment to Washington and hooked it up to the segmented tube. With a charging current of 40 µA and 600 kV on the spheres hooked in parallel (plate 2.3), the tube carried a

[27] Lawrence to Cottrell, 21 Nov 1931 (5/3); Cassen to Lauritsen, 12 Nov 1931 (Lauritsen P, 1/8); Nahmias to Joliot, 21 Sep 1937 (JP, F25), on progress of installation of Lauritsen's Van de Graaff.

proton beam of not quite a millimicroamp (mµA), not enough to burn a hole in cardboard, but enough to make tracks in a cloud chamber. Tuve studied the working apparatus closely and got a spark to his nose for his curiosity. It did not discourage him from reaching higher, for 1.4 MV, half the theoretical value of the maximum potential restricted by the dielectric strength of normal air surrounding a sphere two meters in diameter. (A useful rule of thumb: the theoretical maximum in MV equals the radius in feet.) Simultaneously, Coolidge, supported by GE, planned a high-current version at 1 MV, and Van de Graaff, seconded by the Research Corporation, went forward with one requiring two 15-foot spheres.^[28]

The two-meter sphere, which cost $700, worked well with the million-volt tube insofar as it could be tested outdoors, where it sparked and fluttered under bombardment by bugs and dust and threw lightning bolts that reduced its redwood base to splinters. The Carnegie Institution's Department of Terrestrial Magnetism had no place for the wonder it had built. We read in its annual report for 1931/32: "A highly satisfactory equipment for the production of high energy particles, particularly of high-speed protons, is thus ready for use as soon as operating space becomes available."^[29] During the late fall of 1932, while awaiting the construction of suitable housing off-site (the Carnegie Institution's executive committee approved a building fund in January 1933), Tuve and his associates made a version about one meter in diameter for the space that had belonged to the Tesla coil. With this machine, their desire to do nuclear physics was at last requited. It consisted of two hollow hemispheres of aluminum joined by a short cylindrical section containing the belt, one pulley, an ion source, and the high potential end of a segmental discharge tube. The belt brought about 180 to 200 µA; the sphere held 400 to 600 kV; the tube transmitted as much as 10 µA of proton beam, constant in energy to perhaps 3 percent, to the target. The x rays

[28] Tuve to Lauritsen, 11 Dec 1931 (Lauritsen P, 1/8); Cottrell to Lawrence, 11 Nov 1931 (5/3); Boyce to Lawrence, 17 Nov 1931 (3/8); Fleming to van de Graaff, 19 Dec 1931 (MAT, 8); Tuve, JFI, 216 (1933), 26; Wells, Jl. appl. phys., 9 (1938), 677–80.

[29] CIW, Yb, 31 (1931/2), 230; Tuve, Hafstad, and Dahl, PR, 48 (1935), 317; Fleming to J.M. Cork, 5 Dec 1932, on costs.

incidentally produced drove the experimenters into a hut outside the twelve-inch concrete walls of their laboratory, where they worked by remote control. (Their conservative value for the safe tolerance of radiation was ten times the dose from cosmic rays.) In the operation of the tube, Tuve's group had some advice from Lawrence, whose own investigations had by then acquainted him with the ability of coaxial cylindrical electrodes to focus a beam and with the excellences of certain "Apiezon" oils made by Metropolitan-Vickers as the working fluid of vacuum pumps. With this setup the Department of Terrestrial Magnetism was able to set right some sloppy results reported by Lawrence's Radiation Laboratory.^[30]

In 1933 the two-meter machine found a home and Van de Graaff's 15-foot giant threw its first sparks in a disused blimp hanger in Round Hill, Massachusetts. Tuve's photogenic apparatus (plate 2.4), with four belts and two concentric shells (the inner, one meter in diameter), reached 1.2 MV under favorable conditions. Lawrence visited it and was impressed. "I must say that Tuve's apparatus is performing better than I expected," he wrote the Research Corporation after his inspection. "Seeing Tuve's apparatus perform makes me much more enthusiastic about van de Gr[a]aff's outfit than I was before."^[31] By then, November 1933, the cyclotron could give more volts, but at far less current, than Tuve's "outfit." As for Van de Graaff, his 1.5 MV model gave a charging current almost a million times Lawrence's beam, as his patron Compton liked to observe, and his giant one held promise of another factor of ten (plate 2.5 and fig. 2.9).

"Experience to date indicates that there is in sight no unsurmountable obstacle to the construction of [10 MV Van de Graaff] generators." When Compton spoke these words—which were realized long after the war, by a technology not available in the

[30] Tuve to Cottrell, 24 Jan 1933 (MAT, 4); Cottrell to Lawrence, 16 Sep 1932 (5/3); Tuve to Lauritsen, 11 Nov 1932 (Lauritsen P, 1/8), claiming 800 kV; Tuve to E.W. Sampson, MIT, 22 May 1934 (MAT, 13/"lab letters"), re Lawrence's advice; Tuve, Hafstad and Dahl, PR, 48 (1935), 318–20, 329–32, 337; infra, §4.1.

[31] Tuve, Hafstad, and Dahl, PR, 48 (1935), 321–5; Lawrence to Poillon, 20 Nov 1933 (15/16A). Lawrence based his opinion on Cooksey's negative report on the performance of Tuve's machine in January; Cooksey to Lawrence, 25 Jan 1933 (4/19).

Fig. 2.9
An insider's view of the 15-foot generator. It delivered 1.1 mA to the accelerating
tube under a tension of 5.1 MV. Van Atta et al.,  PR, 49  (1936), 762.

1930s—Van de Graaff's original model was showing off at Chicago's Century of Progress Exposition, "producing millions of volts for the enlightenment of the visitors."^[32] But neither this nor any other million-volt plant was the first to accomplish the purpose of all, and bring that enlightenment to physicists vouchsafed by the disintegration of the atom.

The English Way

Like Van de Graaff and Lauritsen, John Douglas Cockcroft began professional life as an engineer, in his case in 1920, with a degree from the University of Manchester. He spent the next two years as a college apprentice at Metropolitan-Vickers, working with large transformers and strong insulators. And then, like his counterparts, he changed his field and place of study, to mathematics and Cambridge. In his second year there he began to frequent the Cavendish Laboratory, where he did postgraduate work after gaining a second bachelor's degree in 1924. Metro-Vick continued to give him partial support, on the understanding that he would do some research work for them; and, reciprocally, he served the Cavendish as a "spare-time, honorary electrical engineer."^[33]

In 1926 Cockcroft's experimental space was invaded by another man from Metro-Vick, T.E. Allibone, who had been inspired by Blackett's demonstration of disintegration and Coolidge's design for high tension to try his hand at artificial sources. With the advice of colleagues at the High Voltage Laboratory at Metro-Vick, he chose the Tesla coil; and it appears to have been the progress he had made with it during 1926/27 that prompted Rutherford to speak optimistically of the prospects of artificial sources before the Royal Society. Another would-be atom splitter then arrived, Ernest Walton, who came from Trinity College, Dublin, with a degree in mathematics. Walton tried two methods of a type that will occupy us presently. Both failed. Nor did Allibone

[32] Compton, Science, 78 (21 Jul 1933), 50–2; Science service , in Science, 77 (2 June 1933), suppl., 9, quote, on the exposition; Livingston and Blewett, Particle acc. , 65–6, on MIT's postwar 12-MV generator.

[33] R. Spence, DSB, 3 , 328–9; Hartcup and Allibone, Cockcroft , 17–21, 24–5, 31 (quote from Cockcroft), 36.

fulfill Rutherford's hopes, although he did succeed, with the help of a transformer loaned by Metro-Vick, in developing tubes that could stand 450 kV in air and 600 kV under oil. By then, the end of 1928, Cockcroft had finished the research on molecular beams that constituted his doctoral work. Notwithstanding the failures with which his room was strewn, he decided to try to shatter nuclei himself.^[34]

Cockcroft was a businesslike man. He took up atom splitting because he knew it to be practicable. In contrast to all the other would-be splitters, he kept his attention fixed upon the goal: to make particles with energies sufficient to penetrate nuclei, not with energies above a million volts. In estimating the minimum requirement, he followed calculations in a manuscript that circulated in the Cavendish in December 1928. Its author, George Gamow, a young Russian working in Niels Bohr's Institute of Theoretical Physics in Copenhagen, explained that, according to the then new wave mechanics, a charged particle making a head-on collision with a nucleus has a chance of entering even if it does not have as much energy as would be required to do so by the older physics, on which the estimate of millions of volts had been based. Cockcroft deduced from Gamow's equations that protons would be better agents than alpha particles and that a proton of 300 kV would be about one-thirtieth as efficient against boron (in fact, beryllium) as an alpha particle from polonium would be against aluminum. In January 1929 Gamow came to Cambridge to talk, Rutherford approved Cockcroft's project, and Cockcroft and Walton teamed up to produce protons at 300 kV in sufficient quantity to overcome the low probability that any of them would effect a disintegration.^[35]

Cockcroft chose 300 kV as his first goal because he judged it to be within easy reach of the art of vacuum tubes. The arrangement he and Walton devised is shown in plate 2.6. Metro-Vick provided much of the technology: the 350 kV transformer, shown

[34] Hartcup and Allibone, Cockcroft , 38–9; Allibone, PRS, A282 (1964), 447–8.

[35] Hartcup and Allibone, Cockcroft , 40–3; the odds from boron in Cockcroft's calculation of 1928/29 are those for beryllium in Cockcroft and Walton, PRS, A129 (1930), 478. Cf. Hendry, 15–7; Allibone in Hendry, Cambridge physics , 156–61, and in PRS, A282 (1964), 448; Cockcroft, "Development" [1937], 1–2; Gamow, Zs. f. Phys., 52 (1928), 510–5.

schematically at the left, a model custom-made (but later marketed for x-ray plants) to fit the cramped space of the Cavendish; the rectifiers A , designed by Allibone; the pumps, constantly at work on the rectifiers and discharge tube, invented by Metro-Vick's C.R. Brush and run on his Apiezon oil of miraculously low vapor pressure. F is a 60 kV transformer that energizes the little canal ray tube (atop the discharge tube) that Cockcroft and Walton used as a source of protons; the entire transformer F stands at 300 kV above ground. The discharge tube itself, a bulb with two steel pipes as electrodes, terminated in a small experimental space and a hookup to the vacuum system. About 1 µA of 280 kV protons survived the trip down the tube to slam into targets of lead or beryllium. "Very definite indications of a radiation of a non-homogeneous type were found," Cockcroft and Walton wrote in August of 1930, without saying what they indicated.^[36] Very probably they had disintegrated beryllium. Not knowing what evidence to look for—they expected to find gamma rays—they concluded that Gamow was mistaken and sought higher potentials.^[37]

A move to a larger room with a higher ceiling allowed them to build new apparatus to a plan of Cockcroft's. The design multiplies voltages by an intricate set of condensers and switches. In the setup of figure 2.10, where all condensers have equal capacities, the action begins with the closing of the dotted switches, S₁ , S₂ , S₃ . That links K₃ and X₂ in parallel, at the potential E of the constant source. Next the dotted switches open and the solid ones close, causing X₂ to divide its charge with K₂ and bringing each to E /2. The switches are again reversed, leaving K₃ and X₂ at E , K₂ and X₁ at E /4. Reverse connections again: we have K₃ at E , K₂ and X₂ at 5E /8, K₁ and X₁ at E /8. And so on until the upper plate of K₁ is 3E above ground. In practice Cockcroft and Walton used a low-frequency alternating current for E and rectifying diodes as switches; since they could now make rectifiers for 400 kV, and since each rectifier had to stand twice the voltage E , they used a 200 kV transformer as source, four rectifiers, and four con-

[36] Cockcroft and Walton, PRS, A129 (1930), 479–89, rec'd 19 Aug; Hartcup and Allibone, Cockcroft , 43–6; Allibone in Hendry, Cambridge physics , 161–2.

[37] Cockcroft, "Development," 2–3.

Fig. 2.10
Principle of the voltage multiplier
constructed at the Cavendish. Cockcroft
and Walton, PRS, A136  (1932), 620.

densers, in the hope of attaining a final drop of 800 kV. The experimental tube (fig. 2.11) came in two segments; the middle electrode, maintained at half the total potential, carried a little diaphragm, G, to stop stray electrons. The successful proton beam, 10µA at 710 kV, passed from the evacuated tube through a window of mica into the experimental space.^[38] What happened there started an era in nuclear physics.

Metro-Vick continued to play a part. The firm patented Cockcroft's voltage multiplier, although, as it turned out, a German, Heinrich Greinacher, had anticipated him, and only the British observed his rights. Cockcroft's circuit could power a high-potential x-ray plant, and in hard x rays, as we know, there was hard cash. What Metro-Vick had in mind in patenting the voltage multiplier appears from a request from George McKerrud, lieutenant to A.P.M. Fleming, the company's director of research, to Cockcroft, to entertain F.L. Hopwood, a member of the Grand Council of the British Empire Cancer Committee. "Will you please see Allibone and fix up to have a really good show going,

[38] Cockcroft and Walton, PRS, A136 (1932), 619–30, rec'd 23 Feb 1932; Hartcup and Allibone, Cockcroft , 46–7.

Fig. 2.11
Accelerating tube and target arrangement of
the Cockcroft-Walton machine. The source
is at D; C is a metallic ring joint between the
two sections of the constantly pumped tube.
The mica window closes the evacuated
space. Cockcroft and Walton,
PRS, A136  (1932), 626.

because . . . the Cancer Research people . . . are, as you know, a very rich organisation so that we hope that they will contribute towards the support of the work of the future and probably order some tubes to be made." Hopwood left Cambridge convinced that

Metro-Vick could produce "the super x ray tube." McKerrud congratulated Cockcroft on his successful soft sell. "The lunch at St John's was an invaluable detail in the scheme." Hopwood's institution, St Bartholomew's Hospital in London, commissioned Metro-Vick to make it a million-volt x-ray plant based on Cockcroft's patented voltage multiplier.^[39]