356 
DBS. J. AND E. HOPKINSON ON DYNAMO-ELECTRIC MACHINERY. 
mean induction in the magnet to be 105. A wire was taken three times round the 
whole armature in a horizontal plane as at CC ; the elongation observed was 222- 
divisions or 74 in terms of the same units. A wire was taken four times round one- 
half of the armature as at DD ; the elongation was 141, or induction in the iron of 
the armature 7O'5, whence we have 
^T^C 1 ' 05 ' 
It may be well to recall here that m is essentially dependent on the intensity of the 
field, strictly the line B on Plate 19 should not be straight but slightly curved. 
Four coils were taken round the upper pole-piece at EE; the elongation was 159, 
giving 79‘5 on the two sides. Coils at FF give a higher result, 87'5, owing to the 
lines of induction which pass round by the bearings of the machine, and across to 
83 - 5 
the upper ends of the magnets—v 5 is taken to be ^;L = 1T8, 
Kg. 7. 
Efficiency Experiments. 
The method and instruments were those already described, pp. 346 and 347, excepting 
that the current was measured by a Thomson’s graded galvanometer, which had been 
standardised against a Clark’s cell in the position and at the time when used. The 
resistance of leading wires and galvanometer w 7 as 0‘034 ohm, the series coils intro¬ 
duced for compounding the machines were also brought into use, and the losses due 
to their resistance find a place in columns XII. and XIII. of the following Table III., 
in which column I. is lead of brushes of the dynamo, positive for the generator, 
negative for the motor; column II., revolutions per minute; column III., deflection 
of spring in grammes ; column IV., watts by dynamometer ; column AC, volts at 
