4<>2 Mr. B. 0. Peirce on the Spark produced in 



inferior to that obtained when two bright copper wires were 

 separated in the air. 



Rowland has shown that a very powerful electromagnet is 

 not needed in order to obtain good results in studying diamag- 

 netism. In order to see whether a small magnet would not do 

 to increase the length of the spark, a small electromagnet, 

 capable of supporting perhaps two kilogrammes, was set up, 

 and a series of observations taken when the circuit was broken 

 between its poles. In this case the length of the spark was 

 just doubled. Might not a rather small horseshoe electromag- 

 net be advantageously placed upon the Ruhmkorff coil so that 

 the primary circuit should be interrupted between its poles ? 



I next put the electromagnet in one circuit, and broke an- 

 other circuit containing the primary of the induction-coil be- 

 tween the poles of the magnet. With this arrangement two 

 sets of observations were taken under slightly different circum- 

 stances. The results are given below, in the actual space over 

 which it was found that the sparks would just pass. The 

 words " outside " and " inside " denote that the primary circuit 

 was broken outside of the poles and between them respectively. 



Outside. 



Inside. 



2£ niillims. 

 3 „ 



12^ millims. 

 20 



Of course the relative increase in the length of the sparks de- 

 pended upon the relative strengths of the two currents. When 

 the electromagnet is very weak and the outside current very 

 strong, subsequent experiments showed that there was little, if 

 any, increase in the length of the spark. 



As a direct means of showing that the extra effect obtained 

 by breaking between the poles is not mechanical, the following 

 method was employed. The electromagnet was placed in a 

 circuit with two Grove cells. To the end of one of the large 

 wires used for breaking the circuit a very fine wire was fast- 

 ened, leading to one pole of a Thomson's quadrant electrometer, 

 whose other pole was put to earth. Whenever the two large 

 wires were separated, there was of course a deflection of the 

 electrometer-mirror, corresponding to the difference of potential 

 of the poles of the battery ; but beside this there was an in- 

 creased effect when the circuit was broken between the poles, 

 that seems to be unaccountable if we assume the effect to be 

 mechanical. 



When a condenser of about 1 farad capacity was put into 

 the circuit the following deflections were obtained : — 



