breaking a Circuit between the Poles of a Magnet. 465 



At several different times I took a series of observations upon 

 the deflection, by breaking inside the poles and outside of 

 them, when the distance over which the spark had to leap was 

 varied. The curves obtained by laying down these deflections 

 were not, as one might suppose, hyperbolas, but were appa- 

 rently exponential curves, having the axis of x as an asymptote, 

 but not the axis of y. A series of very careful observations 

 was taken by observing the deflections when the distances over 

 which the spark passed were small. The curves obtained by 

 breaking the primary between the poles were similar to those 

 obtained when the primary was broken outside. The observa- 

 tions were taken by breaking the primary with the interrupter 

 of one of Ritchie's induction-coils. The sparks passed between 

 two circular disks of copper 10 centims. in diameter. In the 

 centre of one of the disks was an almost imperceptible protu- 

 berance, in order to ensure the sparks always passing in the 

 same place. One plate was fixed in a horizontal position ; the 

 other was suspended, by a thin ivory handle perpendicular to 

 its plane, to a glass rod placed in the telescope-socket of a ca- 

 thetometer. In this way it was found possible to get the plates 

 sensibly parallel. The poles of the induction-coil were con- 

 nected with a condenser — one directly, and the other by means 

 of the spark passing between the two disks. The condenser 

 was then discharged through a galvanometer. The microme- 

 ter-screw of the cathetometer reads easily to the yooo °f a mu *~ 

 limetre ; and observations were taken with its aid at intervals 

 of "050 millim. The plates were considered to be in contact 

 whenever making the primary circuit gave any deflection in the 

 galvanometer. The zero thus obtained was quite constant, 

 whereas it was almost impossible to tell by the eye just at what 

 point the spark ceased to pass when the circuit was broken. 

 The poles of the battery were kept apart when not actually in 

 use ; and it was supposed that the electromotive force remained 

 constant during the time of observation. 



In laying out a curve, it must be remembered that there was 

 resistance of 6000 ohms already in the circuit. Each of the 

 Following results is the mean of a series of closely agreeing ob- 

 servations : — 



Phil Mag. S. 5. Vol. 1. No. 6. June 1876. 2 I 



