58 J. Trowbridge — .Electromotive Force. 



high tension effects. With the improved form of rheostatic 

 machine which I have had constructed, one-third of a horse- 

 power will produce the effects which have hitherto required 

 the employment of thirty to forty horse-power. The con- 

 densers in this apparatus were very uniform in thickness, and 

 the charging and discharging method was a complete departure 

 from that adopted by Plante. All forms of rotating cylinders 

 carrying contact pieces were abandoned and two lever arms were 

 used to charge the condensers in parallel and then to discharge 

 in series. Great uniformity of action was thus secured. The 

 spark produced by the apparatus have very great disruptive 

 effects and produce a report like the discharge of a pistol. The 

 electrostatic Held in the neighborhood of the terminals is very 

 intense, and the wires leading to the spark gap emit brush dis- 

 charges which extend at least twelve inches from the surface 

 of the wire. I was interested to observe the effect of such 

 powerful discharges through Crookes's tubes, and therefore 

 placed a tube which had been exhausted to such a degree that 

 sparks eight inches in length preferred to pass through the air 

 from terminal to terminal of the tube rather than to pass 

 through the rarih'ed space. The disruptive discharge produced 

 by the machine passed readily through such a tube and showed 

 the X-rays with great brilliancy; moreover the degree of rari- 

 faction of the tube was not sensibly altered by single discharges 

 of such high electromotive force. 



The results so far obtained by the employment of this 

 apparatus prove that the length of spark is proportional to 

 electromotive force. Rarified spaces hitherto considered too 

 high vacua to conduct electricity cease to act like such vacua to 

 very high electromotive force. 



Jefferson Physical Laboratory. 



