DRS. J. AND E. HOPKINSON ON DYNAMO-ELECTRIC MACHINERY. 
339 
Experiment to determine v. 
Around the middle of one of the magnet limbs a single coil of wire was taken, 
forming one complete convolution, and its ends connected to a Thomson’s mirror 
galvanometer rendered fairly ballistic. If the circuit of the field magnets, while the 
exciting current is passing, be suddenly short-circuited, the elongation of the galvano¬ 
meter is a measure of the total induction within the core of the limbs, neglecting the 
residual magnetisation. If the short circuit be suddenly removed, so that the current 
again passes round the field-magnets, the elongation of the galvanometer will be equal 
in magnitude and opposite in direction. 
The readings taken were : 
Zero. 71 left. 
Deflection. 332 ,, magnets made. 
„ 196 right; magnets short-circuited. 
Hence, deflection to right = 267 
„ left = 261 
Mean deflection = 264 
To determine the induction through the armature, the leads to the ballistic galvano¬ 
meter were soldered to consecutive bars of the commutator, connected to that convolu¬ 
tion of the armature, which lay in the plane of commutation. 
The readings taken were : 
Zero . 
Deflection 
5 , ........ 
Hence, deflection to right and left 
23 left. 
223 
176 
178 
200 
,, magnets made. 
j* right; magnets short-circuited. 
It thus appears that out of 264 lines of force passing through the cores of the 
magnet limbs at their centre, 200 go through the core of the armature, whence v 
equals 1 ‘32. The magnetising current round the fields during these experiments was 
5 - 6 amperes. 
Experiments on Waste Field not passing through Armature. 
As in the determination of v a single convolution was taken around the middle of 
one of the limbs, and connected to the ballistic galvanometer ; the deflections, when 
a current of 5‘6 amperes was suddenly passed through the fields or short-circuited, 
were : 
2x2 
