92 BRIDGMAN. 



taneously. The explosive sound, the flash of fire and the pungent 

 smoke are hkely to be rather terrifying on first experience. For 

 nearly all metals the effect can be greatly reduced by a proper design 

 of the die; the best shape of the die varies from metal to metal. 



In spite of the satisfactoriness of the chemical analysis of this 

 sample, it must be recognized that there may perhaps have been in this 

 specimen minute impurities which possibly may have exerted an 

 appreciable effect on the electrical properties. It has been the experi- 

 ence of the General Electric Co. that different samples of calcium, 

 with no perceptible chemical dift'erence, offer Aery dift'erent resistances 

 to extrusion. The effect may not be chemical, and its explanation is 

 entirely obscure. There is no way of knowing whether" the specimens 

 difficult of extrusion are more or less likely to be impure, and no con- 

 nection seems to have been looked for between extrusion and electrical 

 characteristics. The specimen used by me was difficult of extrusion; 

 we cannot now make any use of this fact, but if in the future the cause 

 of the variations should be found, it is well to have the characteristics 

 of this specimen recorded. 



Temperature had to be raised to 400° or higher for successful ex- 

 trusion. Although extrusion would take place at lower temperatures, 

 the wire so formed was brittle, and could be handled only with diffi- 

 cultv. In anv event, the best wire that could be formed had to be 

 handled with care. It broke if bent more than once at a sharp angle, 

 and was entirely different in its properties from sodium. Extrusion 

 took place directly into the air, but as the wire exuded from the die, 

 it was wound immediately onto a spool covered with a protecting 

 paste of Nujol and paraffine. Chemical action by the air is much 

 slower than in the case of sodium and lithium, but is no less complete, 

 the wire eventually crumbling away into dust after standing several 

 weeks. Even the Xujol and paraffine does not act as a complete pro- 

 tection, but there is either direct action or else slow diffusion of the 

 air through the protective coating. In order to avoid error from this 

 effect, the wire must be mounted in the pressure apparatus and 

 measurements made as soon after extrusion as possible, while it is still 

 bright. Unless this procedure is followed effects from the high re- 

 sistance of the surface film are much more troublesome than for any 

 other metal I have tried, not excepting magnesiimi. With fresh wire, 

 however, such effects become vanishingly small with proper manipula- 

 tion. The contact resistance may always jump spasmodically under 

 increases of pressure, but with fresh wire the contact can always be 

 sufficiently restored by the momentary passage of a current from a 

 small magneto. 



