

2IO 



ALTERNATING CURRENTS 



fails when the slip is considerable, since it then becomes impossible 

 to count the rapid revolutions of the stroboscopic disc. The range of 

 the stroboscopic method may, however, be greatly extended by 

 various special devices, one of the most successful of which is that 

 embodied in the direct-reading slip indicator recently devised by 

 Dr. C. V. Drysdale,* and shown in Fig. 137. The instrument 

 consists of a boxwood cone mounted on a spindle whose end may be 

 applied to the motor shaft, so that the cone will be driven at the 

 speed of the motor. Resting on the cone is a pivoted disc mounted 



FIG. 137. Drysdale Direct-reading Slip Indicator. 



at the end of a light fork, which is pivoted in a slider. The position 

 of the slider may be varied by means of a screw, and may be read off 

 on a scale which gives the corresponding value of the slip directly. 

 Attached to the surface of the disc is a stroboscopic disc of paper, the 

 number of black or white sectors being equal to the number of poles 



* The Electrician, vol. Iv. p. 734 (1905). For the use of Fig. 137 the author is 

 indebted to Dr. Drysdale and The Electrician Printing and Publishing Co., who 

 kindly supplied the block for this illustration. 



