J. Trowbridge — Electromotive Force. 57 



Art. IX. — Electromotive Force; by John Trowbridge. 



Lord Kelvin in a paper dated April 12, I860,* entitled 

 Measurement of Electromotive Force required to produce a 

 spark, states that " there is a much less rapid variation with 

 distance of the electrostatic force preceding a spark at the 

 greater than at the smaller distance. It. seems most probable 

 that at still greater distances the electrostatic force will be 

 found to be sensibly constant, as it was certainly expected to be 

 at all distances." 



Having at my command a very much improved Plante 

 rheostatic machine with sixty condensers of 15x80 inches 

 coated surface, I have been enabled to greatly extend my 

 studies of high electromotive force, and to investigate the con- 

 ditions necessary to produce sparks of great body and forty-eight 

 to fifty inches in length. The accumulator which was used to 

 charge the rheostatic machine in parallel had a voltage of 

 20,000, and the machine could then give me 1,200,000 volts. 

 The length of spark which corresponds to the voltage is very 

 closely forty-eight inches. Professor Elihu Thomson, by 

 means of transformers, has obtained sparks of fifty to sixty 

 inches in length, and has estimated the necessary voltage to 

 produce a spark of 80 cm at 500,000. 



This estimate A. Heydweilerf thinks is very much too great 

 and he believes that 100,000 would be nearer the truth. My 

 investigations show conclusively that the estimate of Professor 

 Thomson is far nearer the truth than that of Heydweiler, and 

 instead of being lessened it should be increased. 



In a late paper on the tension at the poles of induction 

 apparatus A. Oberbeck;}; states that a potential difference of 

 60,000 volts under given conditions can produce a spark of 

 more than 10 cm in length. It is difficult to obtain consistent 

 results by the use of induction coils and transformers. 



My results show that Lord Kelvin's conjecture that the elec- 

 trostatic force necessary to produce a spark in air remains 

 sensibly constant for all distances beyond the limit he describes, 

 is correct ; for when the length of spark is plotted as abscissas 

 and the corresponding electromotive force as ordinates a straight 

 line is obtained. Plante calls attention to the fact that the 

 loss of energy resulting from the transformation of dynamic 

 into static electricity is in the case of his apparatus much less 

 than in induction apparatus. This remark applies with great 

 force to the modern use of transformers for the production of 



* Papers on Electrostatics and Magnetism. MacmillaD, London, 1872, p. 258. 

 f Wied. Ann., 48, p. 231, 1803. % Wied. Ann., No. 9, p. 109, 1897. 



