October 9, 1940
Mr. James B. Conant
National Defense Research Committee
1530 P Street N.W.
At our meeting on October 3 W. K. Lewis said that one of the most pressing problems i that of devising an apparatus for indicated the partial pressure of oxygen in the presence of other gases. After visiting the Naval Research Laboratories the next day, Hogness and Latimer told me that this problem had been very strongly presented to them there. I have just thought of a method for doing this.
The one property of oxygen with differentiates it from nitrogen, carbon dioxide, water vapor, and all other common gases, except nitric oxide, is its paramagnetism. The mass magnetic susceptibility of oxygen is several thousand times as great in magnitude as that of those other gases. An instrument indicating the magnetic susceptibility of a mixture of gases containing a significant amount of oxygen would accordingly give the amount of oxygen present, the presence of the other gases causing no significant error. I have designed a device involving a small strong permanent magnet and a pair of small hollow spheres suspended from a quartz fiber, which acts as a torsion balance. The susceptibility of the gas surrounding the apparatus would be indicated by having a beam of light reflected from a minute mirror attached to the fiber passing through a wedge into a photocall, which would then indicate on a dial on the instrument panel. Order of magnitude calculations suggest that it should be possible to build an apparatus of this sort weighing only a few ounces, which would give the amount of oxygen
present at partial pressures of around one tenth to one fifth atmosphere with an accuracy of a percent or two. The susceptibility of oxygen varies inversely with the absolute temperature; this variation might well be corrected by a suitable design of the apparatus, taking into consideration the temperature coefficient of the torsional modulus of quartz.
I feel very optimistic about this idea. There is, of course, the possibility that some serious source of difficulty has been overlooked, but I think that the apparatus can be built successfully, and that it could ultimately be manufactured in quantity at relatively low cost.
Yesterday I sent the following telegram receiving a reply today from Dr. Chadwell giving permission to go ahead: "Have most promising method determination partial pressure oxygen. Best available post-doctorate assistant offered job elsewhere. May I hold him. Please telegraph or telephone." Dr. Reuben Wood, who received his Ph.D. here in 1939 and has been post-doctorate fellow during the year 1939-1940, has said that he will accept appointment as a post-doctorate assistant for national defense work, at $2000 per year, beginning as soon as possible. I set him at work today on the verification of my calculations dealing with this apparatus, so that if the appointment can be made retroactive it might begin on October 9, 1940.
I am enclosing separately information about Dr. Wood, in case that clearance must be obtained for him.
I have the impression that it is desired that an apparatus of the sort under discussion be developed within three months. If there is this need for rapid work, i suggest that a second post-doctorate assistant also be appointed, to help with the theoretical calculations involved.
There is a man available for this work, Dr. Sidney Weinbaum, an American citizen born in Russia. Dr. Weinbaum obtained his Ph.D. in theoretical physics at this Institute in 1933. I would suggest that we not acquaint him with the complete problem, but rather set him to work on individual calculations, such as the calculation of product of field strength and its gradient for different magnet designs. I have no question as to Dr. Weinbaum's loyalty, but I think that if he were appointed he could do his work satisfactorily without knowledge of the nature of the problem as a whole. Dr. Weinbaum would accept appointment at $2000.