144 ON AN EEGOMETER, FOR SMALL ELECTROMOTORS. 
Next, take the angular velocity at a unit distance from 0^ 
then the work due to the above-mentioned forces for one revolu- 
tion will be 
S/r2^. 
Let n denote the number of revolutions per minute of the 
machine, and W the work due to friction in a second; 
then 
60 ’ 
put the value of S fr in (A) into equation B, and we have 
30 
In this way an electro-motor has become its own ergometer. 
If the point C be so arranged as to put in or cut out resistances 
from the circuit, the system may be automatically kept in 
equilibrio. 
By means of a similar method of mounting, a dynamo may 
be made to take the place of a friction brake ergometer, such as 
that of Prong. In this case taking out resistance from the 
circuit will have the same effect as adding pressure in the Prong 
machine. 
