348 
DBS. J. AND E. HOPKTNSON ON DYNAMO-ELECTRIC MACHINERY. 
between them. By suitably varying the magnetic field of one of the dynamos, the 
power passing between the two machines can be adjusted as desired. If, then, the 
electrical power given out by the generator is measured, and also the power transmitted 
by the belt, the efficiency of the combination can be at once determined. By this 
arrangement the measurement, which presents experimental difficulties, viz., the power 
transmitted by the belt, is of a small quantity. Consequently, even a considerable 
error in the determination has but a small effect on the ultimate result. On the other 
hand, the measurement of the large quantity involved, viz., the electrical power passing 
between the two machines, can without difficulty be made with great accuracy. 
The second machine was similar in all respects to that already described, and each is 
intended for a normal output of 110 volts, 320 amperes, at a speed of 780 revolutions 
per minute. 
The power transmitted by the belt was measured by a dynamometer of the Hefner- 
Alteneck type, the general arrangement being as shown in the diagram, Plate 18. A 
is the driving pulley of the engine, B the driven coupling of the dynamos ; D, D are 
the guide pulleys of the dynamometer carried on a double frame turning about the 
fulcrum C, and supported by a spiral spring, the suspension of which can be varied 
by a pair of differential pulley-blocks attached to a fixed support overhead. When a 
reading is made the suspension of the spring is adjusted until the index of the 
dynamometer comes to a fiducial mark on a fixed scale, the extension of the spring is 
then read by a second index attached to its upper extremity; F, F are two fixed 
guide pulleys of the same diameter as the pulleys D, D, and having the same distance 
between their centres, in order that the two portions of the belt may be parallel and 
the sag as far as possible taken up. The normal from C to the centre line of either 
portion of the belt between the pulley A and the guide pulleys=31*9 cms. The normal 
from C to the centre line of either of the parallel portions of the belt=2*4 cms.; and 
from C to the centre line of the spring=92 , 7 cms. 
Take moments about C ; then 
Tension of the belt=;—- X tension of spring, 
= 2 *7 X tension of spring. 
Also the diameter of the pulley B=33*G cms. and the thickness of the belt = 1*6 cm. 
Hence the velocity of the centre of the belt in centimetres per second = 1-845 
X revolutions of dynamo per minute, and, therefore, 
Power transmitted by the belt in ergs per second 
= 2*7 X 1*845 X 981 X tension of spring X revolutions per minute, 
assuming the value of g to be 981. 
