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The mass being free to move is sensitive to changes of velocity and the 
scale may be calibrated to read acceleration directly. The calibration is 
fairly simple and the device is not difficult to construct. 
Another device working on the same principle consists of a ‘“U” tube 
partially filled with mercury so placed that its plane is parallel to the 
motion of the car. It is obvious that changes of velocity will cause the 
mercury to rise in one side of the tube and to fall in the other. The more 
quickly these changes occur the greater will be the difference between the 
heights of the mercury in the two portions of the tube. The tube may 
therefore be calibrated to read acceleration directly. 
Again the accelerometer takes the form of a slightly inclined track 
upon which rolls a ball. This track is made to extend in both directions 
and has a short level portion at the middle. Changes of velocity cause the 
ball to move one way or the other along the track. This device is difficult 
to read and is not very accurate. 
All of these accelerometers are confined to horizontal motions and if 
the track be other than level corrections must be made therefor. This in- 
volves a great deal of labor and expense so that while the devices are 
simple in themselves their use is complicated. It is next to impossible 
to make them self-recording. 
Another apparatus for reading acceleration consists of two magneti- 
cally actuated markers so arranged that dots may be made by each of them 
on a sheet of paper moved at a uniform rate of motion. The magnet of 
one of these pointers has its circuit closed through battery at regular time 
intervals by a clock. The other pointer has its magnet operated on a 
circuit which is closed through battery a definite number of times per reyo- 
lution of the car wheel. From the record made by these pointers the ac- 
celeration at any time may be determined. This apparatus also involves 
a great deal of labor and expense and is seldom used. 
The accelerometer which is the subject of this paper depends for its 
operation entirely upon electrical phenomena and is independent of its 
own location, motion or position. It will therefore read acceleration ver- 
tically or at any angle as well as in the horizontal direction. No cor- 
rections are necessary and it may easily be made self-recording. It is 
not difficult to calibrate and is permanent. 
The circuit as originally conceived is shown in Fig. 3 in which “B” 
is an electric condenser, “C” an ordinary high grade direct current volt- 
