508 WEBER ON THE MEASUREMENT 



C. Repetition of Ampere's fundamental Ex])eriment with com- 

 mon Electricity and ineasurement of the duration of the Elec- 

 tric Spark on the discharge of a Ley den Jar. 



It is evident from the preceding remarks, that the action of 

 a current upon the dynamometer depends more upon the in- 

 tensity of the current, to the square of which it is proportionate, 

 than upon the duration of the current, to which it is simply 

 proportional. Hence it follows that even a small quantity of 

 electricity, when passed through the dynamometer within a very 

 short period, so that it forms a current of very short duration 

 but very great intensity, will produce a sensible effect. This 

 is, in fact, the case when the small quantity of electricity which 

 can be collected in a Leyden jar or battery is transmitted during 

 its discharge through the dynamometer. By this means it was 

 found that Ampere's fundamental experiment, which had pre- 

 viously been made only with powerful galvanic batteries, could 

 also be made with common electricity. 



When the same electricity, collected in Leyden jars, after 

 having been transmitted through the dynamometer, was also 

 conducted through a galvanometer and the deflection thus pro- 

 duced in both instruments was measured, in accordance with the 

 above rules, the duration of the current, i. e. the duration of the 

 electric spark on the discharge of the Leyden jar, and at the 

 same time the intensity of the current could be determined, ad- 

 mitting that the current might be considered as uniform during 

 its brief duration. 



It is well known that in experiments of this kind the discharge 

 of the Leyden jar is effected by means of a wet string, to pre- 

 vent its taking place through the air instead of through the fine 

 wires of the two instruments. In this manner a series of expe- 

 riments was made : a battery of eight jars being discharged 

 through a wet hempen string, 7 milUmetres in thickness and of 

 different lengths, the following results were obtained : — 



Length of the string. Duration of the spark. 



Millimetres. Seconds. 



2000 0-0851 



1000 0-0345 



500 0-0187 



250 0-0095 



Hence the duration of the spark was nearly in proportion to 



