BALANCED MAGNET CHRONOSCOPES 451 



extra pair of binding posts should be mounted, one on the shank 

 of the fork itself, or, as in the figure, on the metal mounting (base) 

 of the fork (fig. 1, electric fork, 3) ; the other on the metal support 

 of the fixed part of the contact. If the fork be connected as 

 described, and then, while it is in operation, one of the condenser 

 connections be momentarily broken, the large increase in the 

 spark at the break of the fork-contact will demonstrate the value 

 of the condenser in preventing burning up of the contact and 

 consequent loss of adjustment. 



The adjustment of the fork-contact is a matter of trial and 

 error. I find it best to adjust to the smallest amplitude of vibra- 

 tion which gives uniform vibration. The contact should be 

 broken when the fork is at rest. It should be hardly necessary 

 to warn against touching the steel of the fork with the hand in 

 starting the fork, but with students this is always necessary. 



Once in operation, the motor should run for hours with no 

 adjustments. If the motor stops, it may be due to one of several 

 causes. (1) Insufficient current. (2) Bad adjustment of the 

 fork-contact, so that the break of the circuit is not sufficiently 

 long. (3) Too tight adjustment of the bearing of the motor 

 shaft. The adjustment is a delicate matter, but once made 

 should be good for a long time. The shaft should not be loose, 

 but should yet spin freely. (4) Too much current in the clutch 

 circuit. Under this condition the jar of the clutch sets the 

 armature "hunting" and it kills itself. The fact is to be empha- 

 sized that the variations in current and contact do not affect 

 the speed of the motor unless they stop it completely. 



STARTING THE MOTOR 



A simple synchronous motor will not start itself: the armature 

 must first be put in rotation at the speed required by the fre- 

 quency of the current, or the frequency of the interruption, and 

 must "get in step" with the current phases. Given this start, 

 the motor will continue to run as long as the current is adequate, 

 until it is overloaded, or until a jar starts it "hunting/ 1 



On the 25 or 50 vibration fork, on 25 cycle alternating current 



