THE INDUCTION MOTOR. 363 



one to the other. In such a case a code of signals for 

 "ready," "on," "ready," "off," must be arranged, the 

 warning " ready " before either applying or removing the 

 counter being essential for reliable working. 



In such a case, if n t and p are the revolutions per 

 second and number of poles of the generator, and n. 2 and 

 p., are the revolutions per second and number of poles of 

 the motor, the slip is given by 



S = n l p l - n, p.,, 

 or when stated as a percentage of synchronous speed 



S per cent. = 100 (HI * " n ^ 



In some cases it may be possible to arrange a gong on 

 the shaft of the generator and a hammer on the shaft of 

 the motor in such a way that the hammer strikes the gong 

 each time the generator shaft makes one revolution more 

 than the motor. If the two machines have the same 

 number of poles, the number of blows per minute divided 

 by 6Q equals the slip of the motor. If the number of 

 poles is not equal, the method is not applicable, since the 

 sounds would be too rapid to be distinguishable. 



Instead of a bell and hammer, discs may be fixed to 

 the shafts of the machines with sectors cut out of them, 

 bearing a definite ratio to the number of poles. By 

 viewing the discs in line with one another, the number of 

 times one disc makes one more revolution than the other 

 can be seen. 



Another simple method is to insert an ammeter in one 

 of the phases of the rotor circuit. The needle of the 

 ammeter will vibrate as the currents induced in the rotor 

 vary. The number of deflections may be counted, and 

 will equal the number of times the rotor makes a half- 

 revolution less than the rotating field. A Morse 

 recorder may be employed to mark the number of 

 changes of current on a strip of paper. By employing 

 a moving-coil direct-current ammeter, the number 

 of vibrations will be equal to the rotor slip, since the 

 needle will only respond to currents in one direction. 

 It will consequently be easier to count the vibrations 

 than if an alternating-current instrument were used, which 

 would give a deflection for the currents induced in both 

 directions. It is hardly possible to count much more than 



