358 APPLIED MECHANICS 
might be useful to make the frame between the pulleys adjustable at d 
and e in order to vary the tension of the dynamometer belt, or better still, 
it might be so constructed that the absolute tension could be noted at any 
. time, whether working or at rest. The working of the apparatus is as 
follows. A driving belt from the source of power is put on the pulley A. 
The machine to be driven is belted from the pulley B. The dynamometer 
belt passes from the lower side of the pulley C to the pulley F, around F 
to D, around D to E, around E back to C.. It will be seen that Cisa 
driving pulley, and D a driven pulley. When the system is at rest, the 
four strands of the dynamometer belt have the same tension. If now C 
revolve and drive D, the tension T, of the belt from C around the loose 
pulley F to D will correspond to the tension of the taut side in a simple 
system of two pulleys, and the tension T, of the belt from the lower side 
of D around F back to the lower side of C will correspond to the slack 
side. 
_ The difference of tension is the driving force P, and taking what 
actually occurs, 
oT, - IT 
<1 = 1, -heaP. 
Now P in pounds multiplied by the speed of the belt is foot-pounds deve- 
‘loped or consumed, ignoring friction. Let r be the radius of the position 
of pulleys E and F from the pivot f. Let / be the distance of the weight 
W from f, to balarice the tendency of the frame K to rotate about / when 
working, then 
Wi=r(2T, - 2T,), or vn =T,-T,=P. 
It is evident that should a Prony brake be put in place of the pulley B, 
the power developed by the motor to A could be determined. Ifa machine 
be driven from the pulley B, the power consumed could also be noted in 
the speed of belt and the position of the weight from the same formula. 
In the use of different belts as dynamometer belts the relative efficiency 
of such belts can readily be determined by the use of the brake attach- 
ment. It will also be seen that only one side of the belt comes in contact 
with the pulleys. 
305. Torsion Meters.—The introduction of the steam turbine, par- 
ticularly for the propulsion of ships, has created a demand for a means 
of measuring the power transmitted by the shaft, since there is no direct 
means of measuring the work done in the turbine. The power to be 
measured is generally very large, and the ordinary forms of dynamometers 
are not suitable or convenient. In the case of the reciprocating engine 
the power developed in the cylinders is readily determined from the 
indicator diagrams, and if these be taken at any given load and also at no 
load, the actual power transmitted by the shaft at the given load can be 
determined with sufficient accuracy for most practical purposes. 
A number of instruments have been designed to measure the ong 
of twist of a given length of shaft transmitting power, and from this 
observation and the speed of the shaft, and certain particulars of the 
shaft itself, the power transmitted is readily computed. 
