464 



Mr. B. 0. Peirce on the Spark produced in 



Length of spark. 



Primary broken 

 outside. 



Primary broken 

 inside. 



1 millim. 

 3 millims. 



22 

 12 



30 

 23 



In a great many cases, where the magnet was quite powerful, 

 it was not easy to get comparable results. When the primary 

 circuit was broken between the poles, the spot of light was 

 driven oft' the scale ; but when the primary was broken outside, 

 there was frequently very little deflection. 



At another time a Thomson's mirror-galvanometer of 6 ohms 

 resistance gave, with an arrangement otherwise the same as 

 the above, the following deflections as a mean of twelve obser- 

 vations : — 



Outside ... 12-9 ■ Inside ... 15*37 



When the electromagnet was in a separate circuit from the 

 primary of the induction-coil, the following observations were 

 obtained with the Thomson's galvanometer. Each result is the 

 mean of about twenty observations. 



Length of spark. 



Primary broken 

 outside. 



Primary broken 

 inside. 



1 millim. 



2 millims. 



3 „ 



4 „ 



21 

 20 

 16-5 

 15-2 



35-8 

 23 

 19-9 

 17 



All these results were taken with interrupting the primary 

 circuit by separating two copper wires held one in either hand. 

 Practice made the results then obtained quite accurate. I at 

 first tried breaking from the surface of mercury ; but, beside 

 the irregularity in the amount of the deflection, there was a 

 most unaccountable change of polarity every few moments. I 

 distrusted the evidence of my own senses so much in this case 

 that I asked several other people to observe for me, without 

 previously telling them any thing of this change of polarity. 

 In each case the observer noticed the reversal for himself. The 

 deflection was almost always in one direction when the circuit 

 was broken between the poles, and in the other direction when 

 the circuit was broken outside of the poles. This rule is not, 

 however, absolute. This reversal of polarity only occurred 

 when copper was used to break from the surface of the mer- 

 cury. The separation of iron from iron, or copper from copper, 

 or iron from mercury, never gave any reversal. The reversal 

 was best seen with the electrometer. 



