BRAKES AND DYNAMOMETERS 359 
From formule proved in Art. 95, p. 80, and Art. 96, p. 81, the 
angular deflection of a shaft in degrees is, _ 180 OMT 0 solid shaft, 
C4 
 andn= SD a for a hollow shaft, where T is the twisting moment, 
EF the length of shaft considered, C the modulus of. rigidity, d the diameter 
of the solid shaft, D and d the external and internal diameters re- 
spectively of the hollow shaft. Evidently for a particular shaft T=—, 
whore / is a constant to be determined experimentally for the particular 
shaft. In the absence of direct experiment on the shaft itself, / may be 
computed by assuming a value for C based on experiments on shafts 
Siietdaofihimilar’ material; thon: e+ *— ‘fora solid. shaft, and 
180 x 32 
oe hata for a hollow shaft. 
If force is in lbs. and linear dimensions in inches, then the horse- 
power transmitted at N revolutions per minute is, 
2eTN _ 2xknN _ kN 
~ 12x 33000 12x 330002 7 
is a constant for the particular shaft. 
Two types of torsion meters will now be illustrated and described, 
the particulars being taken from a paper by Mr. J. Hamilton Gibson, 
read before the North-East Coast Institution of Engineers and Ship- 
builders in January 1908. 
The main features of Fottinger’s torsion meter, which is a purely 
mechanical contrivance, are shown in Fig. 548. A is a dise secured 
Qrk 
, Where ky = 7533000 
U u B 
Fig. 548. 
directly to the shaft. B is another disc secured to the shaft at a distant 
section C through a stiff tube coaxial with but clear of the shaft. The 
motion of these two discs will be the same as that of the points on the 
shaft at which the connections are made, and which are at a distance 7 
apart, and the relative angular motion of these discs will be the angle 
of twist of the length / of the shaft. 
The relative angular movement of the discs is magnified and recorded 
