DRS. J. AND E. HOPKIN SON ON DYNAMO-ELECTRIC MACHINERY. 349 
We may more conveniently express the power in watts (=10 7 ergs per second), 
and write 
Power in watts =‘0004887 X tension of spring X revolutions per minute. 
The potential between the terminals of the generator was measured by one of Sir 
William Thomson’s graded galvanometers, previously standardised by a Clark’s 
cell, which had been compared with other Clark’s cells, of which the electromotive 
force was known by comparison with Lord Rayleigh’s standard. The current 
between the two machines was measured by passing it through a known resistance, 
the difference of potential between the ends of the resistance being determined 
by direct comparison with the Clark’s standard cell, according to Poggendorff’s 
method. As experiments were made with currents of large magnitude, it was im¬ 
portant that the temperature coefficient of the resistance should be as low as possible. 
To this end we found a resistance-frame constructed of platinoid wire of great value. 
The temperature coefficient of this alloy is only 0 ’021 per cent, per degree Centigrade. 
(Proc. Roy. Soc., vol. 38, p. 265 (1885.) 
The resistances of the armatures and magnets of the two machines are as follows:— 
Ohms. 
Generator . 
armature . 
0-009947 
magnets. 
. 16‘93 
Motor. . 
armature 
0-009947 
magnets. 
. 16-44 
The resistance of the leads connecting the two machines was 0‘00205 ohm, and of 
the standard resistance 0‘00586 ohm. 
In all determinations of resistance, the value of the B.A. ohm was taken as 
0‘9867 X 10 9 c.g.s. units, according to Lord Rayleigh’s determination. 
The diagram shows the electrical connexions between the two machines with the 
rheostat r inserted in the magnets of the motor dynamo. 
In order to ascertain the friction of bending the belt round the pulley B, and of the 
journals of the dynamo, a preliminary experiment was made with the dynamometer. 
The combination was run at a speed of 814 revolutions per minute with the dynamos 
on open circuit and the tension of the spring observed—9979 grammes. The engine 
was then reversed, and the dynamos run at the same speed and the tension of the 
spring again observed—3629 grammes. The difference of the two readings gives twice 
the power absorbed in friction, viz. : 1262 watts for the two machines, or 631 watts 
per machine. This is excluded entirely from the subsequent determinations of 
efficiency, as being a quantity dependent on such arbitrary conditions as the lubrica¬ 
tion of the journals, the weight of the belt, and the angle it makes with the 
horizontal. 
