206 



ALTERNATING CURRENTS 



consists simply of two strong square blades which may be bolted at 

 varying distances from the shaft to the cross-arm supporting them. 

 This cross-arm consists of two parts, which may be easily bolted to 

 the shaft of the motor as shown. The power is absorbed entirely by 



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 FIG. 134. Walker's Air-friction Dynamometer. 



air friction, and it need hardly be pointed out that the apparatus 

 keeps perfectly cool. The b.h.p. is calculated from the speed and 

 the position of the blades, but the relation connecting these three 

 quantities is not a simple one, and the results of calibration tests by 

 the makers have to be utilized.* 



121. Eddy=current Brakes 



Various forms of eddy-current brakes have been devised. These 

 are extremely accurate, though more expensive and not so easy to 

 apply as the simple forms just described. They are particularly 

 valuable in cases where very small amounts of brake-power have to 

 be accurately measured. One form of such a brake, designed by 

 Prof. A. Grau f and suitable for small motors, is shown in Fig. 135. 

 It consists of a copper brake-disc mounted on the motor shaft, and a 

 lever, one end of which carries an electromagnet, between whose 

 poles the copper disc rotates, while the other end, which is graduated, 

 carries a sliding weight. A perspective view of the apparatus is 

 given in Fig. 135 (a), while Fig. 135 (5) shows a portion of it in plan. 



* Further particulars regarding this brake are obtainable from Mr. W. G. Walker, 

 47, Victoria Street, Westminster, S.W. 



t Elektrotechnische Zeitschrift, vol. xxi. p. 265 (1900). 



