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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, 1,800 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 haying 24” wheels its 
scale would indicate 0.817 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 yalues 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. 
