360 APPLIED MECHANICS 
by a pencil P, actuated by the system of levers shown, on paper placed 
round the fixed cylinder DE, which is coaxial with the shaft. When 
there is no torque on the shaft the pencil traces a line parallel to the 
_ ends of the cylinder carrying the paper, and this is the zero line of the 
diagram. When the shaft is transmitting power the pencil moves to 
the right or left of the zero line, depending on the direction of rotation 
of the shaft, and at the same is carried round with the shaft, describing 
a more or less wavy line whose ordinates represent the angular deflection 
of the shaft as it revolves. The ordinates of the line traced by the 
pencil are evidently arcs of circles, whose centres lie on the zero line, 
and whose radii are equal to the length of the pencil lever. The cylinder 
carrying the paper can be moved to the left clear of the pencil, and 
the diagram can then be taken off and the mean torque determined. It 
is of course the mean torque which must be used in computing the horse- 
power. In the case of a turbine-driven shaft the torque is practically. 
uniform. 
The Bevis-Gibson flash-light torsion meter depends on the facts that 
the velocity of light is practically infinite, and that light travels in 
straight lines through air of uniform density. Two blank dises A and 
B are fixed on the shaft at a convenient distance apart, as shown in the 
oblique elevation at (a), Fig. 549. Each disc has near its periphery a 
Fic. 549. 
small radial slot, and these two slots are in the same radial plane when 
no power is being transmitted and there is no torque on the shaft. 
(d), (ec), and (d), Fig. 549, are sectional plans, the planes of section going 
through the slots in the discs when in or near their highest positions. 
Behind the dise A is fixed, on, say, one of the bearings of the shaft, a 
bright electric lamp C, masked, but having a slot cut in the mask directly 
opposite the slot in the disc A when the latter slot is in its highest 
position. At every revolution of the shaft a flash of light is projected 
through the slot in the-disc A towards the disc B in a direction parallel 
to the shaft. Behind the disc B, on, say, another shaft bearing, is fitted 
the torque finder D, an instrument fitted with an eye-piece, and capable 
