RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
47 
floor, ljr m. below the level of the bearings of the main shaft, and 4 in. distant 
from the nearest point of the machine. The pump is driven by a small motor of one- 
eighth horse-power and forces the oil through copper pipes to the bearings of the 
machine. After passing through the bearings the oil returns through copper drain 
pipes to the tank and is strained through fine copper gauze before again entering the 
pump. The system works excellently, no trouble whatever having been experienced. 
Section 10.—Regulation of Speed. 
On the axle of the motor a commutator is fitted which serves to charge and discharge 
a condenser four times for each revolution of the shaft. The condenser is placed in 
one arm of a Wheatstone bridge, the other arms of which are platinum-silver resistance 
coils. Balance of the bridge results for a particular frequency only of charge and 
discharge, and to maintain a balance over a considerable length of time the frequency 
and therefore the speed of the Lorenz apparatus must be kept constant. To ensure 
constancy of the arms of the bridge, the condenser and the platinum-silver resistances 
were kept in a constant temperature room ; a small variable resistance in series with 
one of the arms was in general adjusted to secure a balance when the speed was that 
best suited for the resistance measurements, but after the latter measurements had 
commenced the speed only was controlled to maintain the balance. The galvanometer 
used was a suspended coil instrument, the spot from which was received on a ground 
glass scale mounted over the fly-wheel; an assistant observer, S. Watts, applied a 
variable pressure to the fly-wheel and so maintained the balance of the bridge. A 
change in the speed of the Lorenz apparatus of 1 part in 10,000 produced a deflection 
of the galvanometer spot of 4 mm., and in general, a balance was maintained for twenty 
minutes or more with a maximum deflection not greater than 2 mm.; occasionally, 
better results than this were obtained. An adjustable resistance is in series with the 
field coils of the motor, and before attempting to govern the speed this resistance is 
altered until a speed results which is very slightly greater than that desired. 
The motor was run from a battery of large storage cells and very good results were 
obtained with no regulation at all, if the current flowed sufficiently long (generally 
from one to two hours) to raise the field coils to an approximately constant tempe¬ 
rature. However, the method finally adopted enabled resistance measurements to be 
made a few minutes after starting the motor. A somewhat similar method of 
controlling speed for measurements of capacity has been in use for several years both 
at the National Physical Laboratory and at the Bureau of Standards. 
Section 11.—Method of Recording the Speed. 
The chronograph described is very similar to one designed at the Bureau of 
Standards,* and made by the Societe Genevoise, for details of which the writer 
* ‘ Bull. Bureau of Standards,’ vol. 3, p. 561. 
