221 



poor. Its calibration curves are shown in Fig. 11. These imperfections 

 account for the variation in its calibration constant as will be stated later. 

 The resistance circuit contained a three-volt, 1,S00 R. P. M. magneto (perma- 

 nent fields) directly connected to the motor shaft, as was the generator in 

 the condenser circuit. The resistance employed was of the ordinary box 

 type. 



Acceleration was obtained by impressing suddenly a fixed voltage on 

 the driving motor and reading values of speed and the accelerometer every 

 two seconds. Deceleration was obtained by opening the motor switch and 

 reading speed and the accelerometer every two seconds. The speed read- 

 ings were secured by attaching a voltmeter to the three-volt magneto. 

 Some of the readings thus taken are shown in Figs. 5 to 10 which are 

 self-explanatory. 



Scanning these curves brings out their similarity to the mathematical 

 curves on Figs. 1 and 2. 



Calibration is effected by drawing tangents at various points on the 

 speed time curve and dividing the accelerometer reading at this point by 

 the value of the tangent of the angle between this line and the horizontal. 

 This quotient should be constant. Now by noting actual voltage and the 

 corresponding speed the number of volts per revolution may be obtained. 

 Our tangent value indicates volts change in a given time "t" which may 

 now be reduced to revolutions change in the same time. If the gener- 

 ators be belted to a car axle the wheels of which have a known diameter 

 this revolution change may be reduced to the corresponding change of 

 linear velocity in the given time "t." 



For the tests herein described, however, the instrument scale was 

 arbitrarily drawn and, with the particular circuit set up, each small di- 

 vision corresponds to an acceleration of 0.33 revolutions per second per 

 second. If it had been used on an interurban car having 24" wheels its 

 scale would indicate 0.S17 feet per second per second per small division. 

 This value could be reduced to a workable figure by using a larger con- 

 denser, a higher voltage and a more sensitive voltmeter. 



These calibration values varied from 15 to 25 revolutions per second 

 per second per small scale division on account of imperfections in the in- 

 struments and the small readings made necessary by having insufficient 

 capacity. 



