320 KNIGHT DUNLAP 



interrupted, causing the drum to be positively arrested and 

 rigidly held, through the attraction of the disc H 2 to the non- 

 rotating magnet core G z . 



The worm-wheel on the present instrument has 150 teeth, and 

 hence when the motor is driven by a 25 vibration fork, the drum 

 makes one rotation in one minute, if the fork and motor are in 

 simple step. However, by starting the motor at double speed 

 (either speed may be secured by a twist given to the armature- 

 shaft by the thumb and finger) it may be made to run at that 

 rate, two poles passing for each current-interruption, and the 

 drum makes one rotation in thirty seconds. Forks of different 

 rates may be used, up to 100 vibrations per second, although 

 ordinary forks above 50 in frequency do not make good contact. 

 Driven by the 60 cycle alternating current, without fork, but with 

 suitable resistance, the drum makes one rotation in twelve and 

 a half seconds. If a rectifier, of the "Tungar" type be used 

 instead of resistance, the drum makes one rotation in twenty-five 

 seconds. The motor runs well on alternating current of 

 frequencies of from 15 to 120 per second. 



At the slower speeds: with 25 and 50 vibration forks, or with 

 the 60 cycle A. C. with rectifier, or 25 cycle A. C. without 

 rectifier: the method of starting the motor described above 

 (thumb and finger) is the best. For the higher speeds; with 60 

 cycle A. C. without rectifier: a different method must be used. 

 A single layer of adhesive tape (electricians, or surgeon's, is 

 wrapped smoothly around the armature shaft, and the motor is 

 started by drawing the fingers smartly across the shaft. This 

 method of starting is easier if the current is off the motor-field, 

 and is put on by closing a switch at the moment when the right 

 speed is obtained. In any case, the starting is a " knack' 7 

 which should be readily learned, after which the process is 

 simple. 



With any method of motor drive, any speed witnin reasonable 

 limits may be obtained by using a worm wheel of the requisite 

 number of teeth. Each size of wheel requires, of course, a 

 specific location of the motor on the base, so that the kymo- 

 graph should be built for the speed of drum required. It is 



