93 
1879.] E. Schwendler —On 'Electric Light Measurements. 
All resistances to "be expressed in S. U. and the temperature to he sta¬ 
ted at which the measured resistances are right. 
As these resistances are all very small, they should he given to 3rd 
decimal; hence a special bridge for testing must be used. 
11. The speed of driving is to be between 700 and 750 revolutions 
per minute, and at this speed through a known external resistance, the 
current produced should not he less than that given by the following 
formula :— 
where C is the current in Webers. 
W l the power required per second to produce that current. 
w 1 the power per second to drive the dynamo-electric 
machine 
empty (external resistance infinite, i. e., circuit open). 
Both W 1 and w 1 are expressed in Meg Ergs* per second. 
1 Meg Erg = 1,000,000 Ergs. i ; . 
m is the internal resistance of the- dynamo-electric machine, i. e., 
resistance of electro-magnet plus resistance of induction cylinder plus 
contact resistance in the commutator as found by direct measurement 
when all brushes are on. 
r is the external resistance ; both m and r expressed in S.U. 
When gauging the dynamo-electric machine by the above formula 
it is best to make r = 1 S.U. about. 
If the dynamo-electric machine produces less current (say more than 
10 per cent, less) then there is something wrong either in the machine or 
with the measurements. If it is in the machine it may be due to the 
following causes :— 
Brushes make bad contact and do not approach sufficiently near to 
the neutral line ; try therefore a better adjustment; or there is a shunt in 
the machine which can best be found by driving the machine empty ; or 
the iron is bad, &c., &c., &c. 
The actual measurements are to he forwarded with the dynamo- 
electric machine. 
Mote. — The above formula has been calculated from the experimental 
fact that at the production of currents above 20 Webers, the loss of power 
in making current is about 12 per cent. If all the. force used for pro¬ 
ducing current were represented in the external circuit by that current 
flowing through a known resistance, then the formula would be : 
* _ 7460'6 Meg Ergs per second, equal one Watt’s H.P. per second, i. e., 550 foot¬ 
pounds per second in London. 
