1877.] 79 (Draper. 
way from one pair of brushes when the Gramme bobbin is making 1200 
revolutions per minute is equal to 100 candles, and is greater in quantity 
and intensity than one would like to send through a valuable induction 
coil. I usually run the bobbin at 622 revolutions per minute, and this rate 
will readily give 1000 ten-inch sparks per minute with the 18 inch coil. 
Of course a Pliicker’s tube lights up very vividly and generally, in order 
to get the maximum effect I arrange the current so that the aluminium 
terminals are on the point of melting. The glass, particularly in the capil- 
lary part often gets so hot as to char paper. The general appearance of 
the machine is shown in Fig. 1. 
cet 
Fie. 1.—THE GRAMME MACHINE. 
As long as the Gramme bobbin is driven at a steady rate the current 
seems to be perfectly constant, but variations of speed make marked differ- 
ences in the current and this is especially to be avoided when one is so 
near the limit of endurance of Pliicker’s tubes. A reliable and constant 
motor is therefore of prime importance for these purposes. <A difference of 
one per cent. in the speed of the engine sometimes cannot be tolerated, and 
yet at another time, one must have the power of increasing and dimin- 
ishing the rate through wide limits. The only motor, among many I have 
examined and tried, that is perfectly satisfactory, is Brayton’s Petroleum 
Ready Motor. 
