William D. Coolidge

dental radiology Editor: LINCOLN American

R. MANSON-HING, D.M.D., Academy of Dental Radiology

School of Dentistry, University 1919 Seventh Avenue South Birmingham, Alabama 35233

M.S.

of Alabama

William D. Coolidge Reflections

T

of one hundred

years

his year marks the one hundredth birthday of one of the most distinguished members of ‘the Academy of Dental Radiology (Fig. 1). Dr. Coolidge was born on Oct. 23, 1873, and the academy sends its best wishes for a happy birthday. Much has been written over the years about Dr. Coolidge. The best of these reflections are the ones written by Dr. Coolidge himself. For example, a letter written to his good friend Dr. Louis Amyot and published in an article entitled “From My Earlier \-ears; Reminiscences by William D. Coolidge” in the Bulletis of the Histor:~ of De?ltistry (,June, 1972) states: “It’s interesting to look back over the path of one’s life and to seeits many direction changes, often radical, although due to events which seemed,at the time, almost trivial. When I look back, I seethat first of all, of course, I inherited much from my parents . . . long life for example. Father lived to be 94, had good health all that time, went to sleep one night and didn’t wake up in the morning. Mother lived to be 85. Father worked in a shoe factory. We had, at home, a seven acre place devoted largely to the raising of apples and peaches, with a little devoted to the family vegetable garden. Mother added somewhat to the family income by taking in (Ir~ssmaking. I was an onI\- child, but the family income did not suggest my going to college. In grade school, I hated to get up before the class and ‘speak a piece.’ My father, knowing this, arranged for me to take some private lessonsin elocution. The young lady teacher was very good. She must have been, for she made what had been torture for me almost a pleasure. I think that I still feel the effects a little. In my home-town a Mr. Frank Knight had a small machine shop and allowed me to make small gadgets there for myself, using his tools. He often showed me how to do a better job, and gave me an active interest in things mechanical and electrical. His son, (George, a classmate of mine in high-school, suggested my applying for a state scholarship for M.I.T. I did so, and was a,warded one. It was in the freshman chemical laboratory at M.I.T. that I first met Willis R. Whitney, my teacher there who was subsequently to exert the 592

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Willimt

36 4

Kg.

1. William

David

D. Coolidge

593

Coolidge.

strongest influence of anyone, on my life. While a student at M.I.T. during the last year of my electrical engineering course, I one day asked the physics teacher, Prof. Goodwin this question : supposing I had a drinking glass half-full of water and that I added to the water a substance such as sugar, for instance, which dissolved in the water, what would happen to the height of the liquid level in the glass? To my surprise, he said that some added substances would raise the level, while others would lower it! This took my mind somewhat away from engineering, as it seemed more fundamental. Following this experience, an M.I.T. classmate in our last year, suggested that I apply for a fellowship for advanced study abroad. This application was made and granted and resulted in my studying physics, chemistry and geology at the University of Leipsic for over two years. Many other events made a marked change in my lifeline, but I remember one in particular. I had gone to a Dr. Pettengill, a dentist in Hudson, Mass., my home town, for the filling of a cavity in one of my teeth. As I watched the preparation of the filling material, silver amalgam, I was much interested and impressed by its physical properties. Made from liquid mercury and metallic silver, it was a somewhat sticky mass, plastic over a considerable temperature range. Scars later the memory of that material led me to use an amalgam, this time cadmium and mercury, in the production of squirted tungsten filaments for the incandescent lamp. Tungsten is a metal which had always been as brittle as

glass; but these filaments taught us how to make it tluctiltt ant1 as strong as steel ; a11t1 ilr this form it is ustttl for incantleswllt. lamps ant1 other purposes, and has 1~1 to the prcscnt S-Y~J- tube. This has all followed from having that tooth filletl.” Tho ahilit)- to work the metal tungsten ma& possible the moclern x-ray tube. The ercnt is best tleswiht~tl in an article written 1)~ Dr. Cooliclgc in 1963 for the Netallurgical Society of the American Institute of Mining, Metallurgical anct Pctrolcum Engineers. The artielc, appropriately entitleti “The Development of 1)uctile Tungsten,” was published in The SorDy Ce?~texGaZ S’ywI)o,sizlnL ON thp Ifistory of ,Il~!tallwy~y, Volume 27, 1963, and is reproducctl hwt: I\-ith the kintl permission of the Metallurgical Society of AIME : The question may wclI 11th asked how it happened that the discovery :~nddevelopmentof duct&: tungsten \VVK made by t.he General Electric Company, and how T, rather than a metallurgist, happen4 to have a leading role in this. As I see it now, fifty-odd years after the event, it was due, among others, to the following circumstances : 1. Dr. Willis R. \\‘hitney had recently established for the Company a pioneer industrial laboratory for fundamental research. 3. The Company had :I large incandescent lamp business. :i. One of the early contributions of the Laboratory was Dr. Whitney’s substantial improvrmcbnt in the carl)on filament of that lamp, which emphasized the possible fruitfulness of research in the incandescent lamp field. 4. German work on metal filaments of osmium and, especially, tantalum, had threatened the competitive position of the carbon filament, thus justifying much resr:~rch in the metal filament field. Kn 1905, when 1 joined tllct staff of the Research Laboratory, work was there in progress ou several different methods of making tungsten filaments by extruding the powder of the metal, or one of its compouncls, mixed with starch or other organic binder, through a die into :I thread which was subsequc~ntly heated in a suitxblc atmosphere to remove everything but the tungsten, and then sintered. As a. result of the organic binder, these finished filaments containctl traces of carbon which, in :L lamp, slowly evaporated and condensed on the inside of the Imll~, thus reducing thrl lamp efficiency. At the time, we knew of no way of freeing such filanx*nts from c:~rl,on traces Ilefore putting them into lamps. We learned later that a way to do this had IKYW found by Messrs. Just and Hanxman in Vienna, U.S. Patent No. 1,018,502. ‘t’hc~~ was not only this trouble, but the filaments were brittle, thus making the lamps very f rngile. Although in the light of past experiences, tungsten had always been brittle, it seemed possible that tllis brittleness had bcc~n due to impurity, and was not characteristic of the pure n&al. As the tungsten we were using had received much chemical purification, and as absolute purity is n1~1gs unnt.tainablc, it seemed well to st.udy the effect upon brittleness of adding differrnt foreign elements. For such t(lsts, it s~mc~~ ljrst not to use an organic hinder, but rather to press the dry tungsten powder, with or without added foreign admixture, into small rods, melt an end of coach rod in v:~cuum or inert atmosphere, and then hammer this end at some lower temperature. Tn the early months of 1906, we mxde such rods about $iG inch square and 2 inches long and, to permit handling, partly sintered them by firing in hydrogen. Serving then as an electrode in 3 small mercury-arc furnace, developed for the purpose, each rod was slowly heated up to a tc~mperaturo at which a melted globule formed at its upper end. During this operation, it could lx seen that most, of the added materials vaporized out from the rods. On the following (*rude hammering test, the glolmlcs and the adjacent well-sintered parts were all brittle cold; lmt, at, red heat, they were quite malleable. We saw no difference in malleability between the

Volume Number

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Tl’illia~~~ D. Coolidge

595

rods with, and those without, the additions. From this, we concluded that further purification of our tungsten from those elements which we had added would not increase ductility. In the course of these experiments, we had observed that if that portion of a rod which had not been fully sintered in the mercury-arc furnace were, before air was admitted to the furnace, brought into contact with the liquid mercury of the furnace, the pores of the rod would fill with mercury. This suggested that partly sintered rods could, in this way, have their pores filled with ductile amalgams, as of cadmium, tin, lead, etc. It seemed possible that this might lend sufficient ductility to permit the rods to ho rolled and drawn into wire, from which tho amalgam could be removed by vaporization in vacuum. We succeeded by this method in tilling the pores with various amalgams, but did not get sufficient ductility. The porosity of the rods could, apparently, not be made sufficient to accommodate the needed amount of amalgam. The same applied to other experiments carried out later, in March of 1906, in which the pores of the rods were filled with one of several pure metals, including cadmium, silver and gold. These last resulting rods would l)e rolled somewhat, but still lacked enough ductility to be interesting. In the course of these experiments with impregnated rods, we had become acquainted with cadmium amalgams, and the long temperature range through which they are plastic. We found, for example, that a 50 per cent cadmium amalgam could be extruded under pressure, from a heated mold, through a die, into nice smooth wire. We also found that if this amalgam were heated up to its plastic temperature range in a mortar, we could, with a pestle, rub tungsten powder into it, and that even after getting in more than 50 per cent by weight of tungsten, we could, at a lower temperature, squirt it through a die into a pliable, strong, smooth wire. It also developed that, by passing current through a loop of this wire in a vacuum, the mercury would vaporize out first, and then, with rising temperature, the cadmium, to be followed by further temperature rise, with complete sintering of the tungsten into a smooth shiny filament. With minor modifications, such as the addition of a little bismuth to the amalgam, this became the so-called Amalgam Process, and was used in our first commercial production of tungsten lamps, U.S. Patent No. 1,026,343, granted May 14, 1912. It yielded filaments free from carbon and also free from mechanical defects. Such filaments not only made good lamps, but they also served as the starting material for our first drawn tungsten wire. We found that such amalgam process filaments were bendable (permanently deformable) even below red heat, and could even be coiled around a small platinum mandrel into tight spirals. Early in 1907, we made experimental lamps with such spiral tungsten filaments. The ease of bending these amalgam process filaments, and at so low a temperature, led to various experiments made in the hope that hot-working might increase the strength of the metal. We pressed them between hot blocks of special steel, and found that tungsten ribbon produced in this way could be cold bent (not permanently deformed) into an arc of smaller radius than other pieces of such ribbon which, after pressing to the same thickness, had been heated to a high temperature to remove the effect of hot pressing. We also successfully passed such filaments between the heated steel rolls of a small rolling mill. The first tungsten filament to show that it eouid be permanently deformed at room temperature was a 9.8 mil amalgam process filament which had been drawn through a series of five heated diamond dies. In later experiments, we were able, starting with these same amalgam process filaments, to hot-draw them down through a series of progressively smaller diamond dies, to produce tungsten wire in lengths of many feet, and of very small diameter and great tensile strength. The wear of the dies was at first excessive, but was later reduced by the use of a graphitic lubricant. For the commercial production of tungsten wire for lamps, it was, of course, desirable that me be able to start with something larger than our amalgam process filaments, having a diameter of only some twenty mils. We, therefore, undertook to make % inch square rods by pressing dry tungsten powder with no binding agent. Under high pressure, laterally applied, and with sufficiently rigid molds, we were finally successful in making good ingots, free from pressing defects. These were then partly sintered by heating them in a tube-furnace with a

hjYlrogtw :ltll1os]~ll1~1 (1. ‘I’hlsy \rvw then tr:itlsi’vrrrvl 10 ;I trwtinfi ?lr:rrnlwr ill which thv ,I,‘IH’~ 1x1111of’ tllv rot1 \ws hc~lil iii :I statiourrt~y ck~rnp, \~llill~ tltr lower cntl c:lrricvl 2 vnplwr clnni~) wllir~h, to permit shrirlk:lp(~ of tllc> rotl, was swirnr~~ing in a pool of’ ~wrcury. 1’11,> rod :cnrl the ci wuit tc~rminalx to which it \\:rs :rtt:rc*hc~l wore c~nclo.st~ti in a metal clurrnb~~r tilled with flolving hytlrogcn, nntl the heating current \!-a’; gratlunlly raised to llring the rod almost to the melting 1mint. \Vo tricct Ilot-ll;lrrirllrriIrg suc~h rods, first 1,~ hanrl, and thaw with a snmll po\~c~r-llnlllrrifr. Sorncs 1110~s could 1~ struck without cracking, lmt others not. J\ skilled 1)l:tcksmith from the \\‘orks WRS fine-grxined tungsten rods, during heating in hydrogen, quickly lost. their oxygen and then sintcred, thus closing their pores before enough hydrogen could get to the central portions to free them of their oxygen. Drastic efforts by greatly prolonging the hydrogen treatment at lower and gradually increasing temperatures all resulted in mechanically unworkable rods. Tn 7910, having satisfied ourselves that we must use tungsten powder of sufficient coarsencss so that the rods would retain t,heir porosity at temperatures permitting complete removal of oxygen throughout the rods, before appreciable sintering took place, we concentrated on the problem of consistently making sufficiently coarse tungsten powder. The method adopted consisted in the high temperature firing of the tungstic acid, from which the tungsten was made, in a Hessian or Battersea. crucible. Upon reduction, this oxide yielded sufficiently coarse tungsten powder. As the rod increased in length in swag@, it became necessary to pass it through a

Volume Number

Fig.

William

36 4

8’. Plaque

affixed

to the Coolidge

Laboratory

of the General

D. Coolidge

Electric

597

Corporation.

furnace placed in front of the swager, and to use feed-rolls at the rear of the swager. In this way, the diameter was reduced to the point where it could be hot-drawn through diamond dies. The wear of the diamond dies was at first great enough so that we seriously considered the practicability of hot-rolling the swaged wire down to lamp-filament sizes. With a tiny mill having steel rolls only a half inch in diameter, we did succeed, in 1910, in experimentally rolling tungsten wire down to 5.7 mils square. By this time, however, we had learned how to reduce die wear sufficiently. Bcforc adopting the practice of firing tungstic acid in a Battersea crucible, to get coarse tungsten powder, w had found that lamp filaments made from our drawn tungsten wire offset I~ndly~, and so had short life, when operated on alternating current. This did not apply, howWY, when the Battersen crucible had been used. This had been predicted from microscopic examination of fractures of rods which had been swaged, and then heated to a high temperature. Without Battersen-firing, the fracture would often show large crystals, while, with the Battersea, the fracture was relatively fine-grained. Refractory oxides had been introduced by the cruciljle from its hot walls, and these impurities carried over into the tungsten, where their presenro interfered helpfully with crystal growth. We found that rare earth oxides, such as thorin, for cxnmple, added to tungsten powder, in whose preparation no Battersea crucible had been used, also prevented offsetting, as did zirconia and ceria. Powder metallurgy had, then, made possible the control of crystal growth that one would not have had if the starting rod had been a melted ingot. Wr later, in 1911, increased the size of pressed tungsten powder rods to as much as an inch square and 8 inches long, and swaged these into cylindrical shape for X-ray-tube targets. We also rolled these rods into sheets from which we hot-punched discs for other X-ray targets ;lnd for automobile ignition make-and-break contacts. In the mechanical working of tungsten, temperature plays a dominant role. Up to around 1500” C mechanical working tends to change the crystalline to a fibrous structure, thus imparting strength. Above this temperature, fibrous tungsten tends to revert to the crystalline condition. The best temperature for working will then depend on the strength required in the finished product. If working is carried too far at low temperatures the metal loses strength through the lateral separnt.ion of its fibers. The higher the temperature, the softer is the metal;

an11 whc~re mwh rotlwtion of cross section is to 1~ accomplished, it may be desiral)le to work first at, a temp<%r:tture higher than 1500 degrees, where the metal is relatively soft, ant1 then to c~lmnge to lower temlwratures for fiber produrtion and strength. 1’11~ research and development work leading finally to tho commercial production of ductile tungsten for various purposw was made possil~lo through the close co-operation of many General Electric scientists and engineers-so many, in fact, that it would be impossible for me to name them all and properly assign credit. There was also help from others outside the, Company. As an ex:tmple of this, take the easo of the diamond dies for filament squirting and tungsten wire drawing. 1t earlg became clear that Ive must, ourselves, learn to make diamond dies, shaped and mounted specifically for our tungsten work. Through the Waltham Watch Company, we learned of an clxpert lapidary, a Mr. Palmer, who came to us and stayed a month, teaching us something of the lapidary art. As another example in this same field, we learned from Mr. C. A. Cowles, of the Ansonia Brass and Copper Company, about their use of diamond dies in copper wire drawing. We were, at the time, eager to know what was the largest size of diamond die that we could afford to use in drawing tungsten, as this would determine the smallest size to which we must wage or roll. I remember Mr. Cowles showing me some very large diamond dies and offering to lend them to us, but that they looked to me too fragile and expensive for us to borrow them. Sinrc adoption of the Laboratory ductile tungsten process, by the Company’s Incandescent Lamp Division, some changes in the process have been made by the Division, but it is still one of the Company’s most important applications of powder metallurgy. The successful outcome of our ductile tungsten work was due to the production of the right microstructure. I must, however, say that we were guided, in the main, 11y the experiment itself, rat.her than by metallurgical knowledge.

The x-ray output of the early gas tubes was very unpredictable, with the tubes sometimes requiring re-evacuation and replenishment with gas. The turning point came in 1913, when Dr. Coolidge unveiled the first successful highvacuum x-ray tube. The design provided stable tube operation and easy control of x-ray quantity and quality. The first commercial Coolidge tube was rated at 140 KVP and 5 Ma. for continuous operation. In 1927, Coolidge produced a 200 KVP, 8 Ma. air-cooled therapy tube, and in late 1922 he reported on a 200 KVP, 30 Ma. water-cooled tube. The new tubes were developed for the deep-therapy x-ray units. Dr. Coolidge has received many awards, among which are the Rumford, Hughes, Faraday, Duddel, and Franklin medals. Dr. Coolidge has spent a lifetime of work at the General Electric Corporation and is Director Emeritus of the Research Laboratory. In 1948, the Company dedicated the Coolidge Laboratory, and t.he plaque affixed to this building is shown in Fig. 2. Today, Dr. Coolidge, your fellow members of the American Academy of Dental Radiology wish you a most happy birthday. This article was made possible through Schenectady, New York, and Mr. Jack Astbury cooperation is much appreciated.

the of

assistance of Dr. Louis B. Amyot of the General Electric Corpora6on. Their

Lincoln tc. Munson-Hing, D.M.D., Editor, American Academy of Dental Radiology

M.S.