418 BELL SYSTEM TECHNICAL JOURNAL 



under others it is oscillatory, but damped. This beautiful bit of mathemati- 

 cal analysis, exact and thorough as it was, and clarifying the entire phe- 

 nomenon, passed almost unnoticed at the time; but it came into its own 

 with the arrival of wireless telegraphy. 



There followed a few years later a direct experimental verification of the 

 theory of the oscillatory nature of the Leyden jar discharge. In 1858 

 Feddersen^ examined the spark by means of a revolving mirror, and extended 

 his researches during the following year by the use of photography. There 

 was thus obtained visual evidence of the reversal of direction of the dis- 

 charge, and it was even possible to determine the frequency; photographs 

 of these oscillatory sparks were sent to Thomson, who had suggested in his 

 paper this very possibility of proof. Other experimenters followed with 

 variations of this method of investigation, and in 1890 Boys^ improved 

 upon it by photographing the spark by means of a series of rapidly revolving 

 lenses. Shortly before this a very important discovery had been made in 

 connection with the spark discharge, Hertz's discovery of electric waves, 

 and as a consequence more physicists were turning to a study of its char- 

 acteristics, chiefly with the aid of photography. 



The Effect of Capacity in an Alternating Current Circuit 



It will be noted that the foregoing account is concerned with the very 

 rapid, and transient, motion of electricity in open circuits. While knowl- 

 edge of this sort of electric current was being advanced, Faraday's (and 

 Henry's) discoveries in electromagnetic induction had made possible the 

 invention of the dynamo and the production of a sustained alternating 

 current — ordinarily of much slower motion. This generator, at lirst in 

 the form of the feeble magneto, was for a long time not much more than a 

 toy, and experience continued to be limited largely to the direct "galvanic" 

 current. When eventually alternating currents began to be employed to an 

 appreciable extent, in experiment and in industry, there were some new 

 phenomena encountered, and those less theoretically grounded were slow 

 to realize the peculiar effect of a condenser in the circuit, although the 

 choking effect of an inductance alone was easily apparent. 



The name of the great genius Maxwell now comes into our history. As 

 we have seen, Lord Kelvin was the first to give a mathematical treatment 

 of the oscillations of a Leyden jar discharge. So Clerk Maxwell was the 

 first to publish an analysis of the effect of capacity in a circuit containing 

 inductance and resistance and an impressed alternating electromotive 

 force, and to show the conditions for resonance. The way in which he 

 came to solve this problem makes an interesting story, and it was told in a 

 characteristically interesting manner by the late Professor Pupin in the 



