68 BRIDGMAN. 



lithium wire, but because of clanger of oxidation, the lithium has been 

 usually enclosed in a glass envelope. Measurements in glass, such as 

 that of temperature coefficient of resistance for example, are not 

 entirely free from objection, because of the constraining action of the 

 glass walls. The error introduced by effects of this sort is presumably 

 not large, but it is nevertheless gratifying to be able to avoid it. 



A somewhat special technique is necessary to handle the bare wire 

 successfully. It must, of course, be protected at all times from direct 

 contact with the air. This was accomplished at first by extruding it 

 directly into melted white vaseline, and winding it directly from the 

 pot of vaseline onto the bone core, which was mounted for slow rota- 

 tion by hand. The wire, in passing from the pot of vaseline to the 

 bone core, becomes covered with a capillary film of vaseline, which 

 rapidly solidifies in the air, forming a perfectly protecting coating. 

 Later, however, the vaseline was replaced with a mixture of "Nujol," 

 which is a carefully refined heavy hydrocarbon, prepared by the 

 Standard Oil Co. and sold as a remedy for constipation, and refined 

 paraffine, melted together in such proportions as to have about the 

 consistency of vaseline. This mixture was suggested to me by Dr. 

 Conant of the Chemistry Department. This change was made 

 necessary by the chemical action of the vaseline. The vaseline of 

 commerce is not a substance of standardized properties; the first can 

 of \'aseline which I tried was without appreciable chemical action, but 

 the second was unpleasantly corrosive in its action. The mixture of 

 Nujol and paraffine was much more satisfactory, the lithium remaining 

 bright for days. 



The liquid transmitting pressure must also be chosen with care in 

 order to avoid chemical action. At first I used commercial kerosene 

 which had been standing in contact with sodium for several weeks. 

 This was not satisfactory, however, the kerosene gradually turning 

 yellow at room temperature in contact with the sodium, and at higher 

 temperatures the reaction is much accelerated. The transmitting 

 medium finally used was a mixture of Nujol and "petroleum ether" 

 in different proportions, depending on the temperature of the work. 

 At 0° nearly piu'e petroleum ether must be used in order to avoid 

 freezing under pressure. Another source of chemical action, besides 

 the transmitting medium, is the bone core on which the wire is wound. 

 It is necessary to drive the water as completely as possible out of the 

 core by prolonged heating to 130° or so, but under these conditions the 

 bone becomes very brittle and must be handled with extreme care. 

 Even after every precaution had been taken, some chemical action still 



