213 
to speed, and even if the voltages were constant—acceleration zero—the 
instrument could not return to its zero position. 
The circuit must therefore be modified to compensate for this small 
leakage current, as is shown in Fig. 4. A second direct current magneto 
(or another commutator on the original machine) is arranged so that it 
ean feed current through a high resistance to another coil on the moving 
element of the instrument. This second coil is wound over the first and 
works in the same magnetic field. The current is passed through it in such 
a direction that the torque produced thereby opposes the torque of the 
original coil. By adjusting the high resistance these torques may be made 
equal and the instrument will read zero for any constant value of voltage 
within reasonable limits. This allows the charging currents to actuate 
the instrument entirely independent of the leakage current and condensers 
of reasonable cost may be employed. 
In Fig. 4 the second generator is shown at A,, the high resistance at 
R, and the second coil on the moving element of the instrument has its 
terminals shown at T, and T.. These terminals are also sbown in the 
separate sketch of the instrument ©. It will be noted that the pair of 
magnetos are shown belted to a car axle. When this is done changes in 
the rate of motion of the car will produce changes in the voltages of the 
magnetos so that the instrument may be calibrated to read accelerations 
in terms of feet per second per second, as well as in terms of revolutions 
per second per second. 
Figures 5 to 11 show the results obtained recently from tests on this 
type of accelerometer. Three curves (Figs. 5, 6 and 7) show positive ac- 
celeration, and three (Figs. 8, 9 and 10) show negative acceleration. 
The experimental apparatus with which these results were obtained 
was made up as follows: the direct current machine in the condenser cur- 
cuit was a separately excited generator of about 500 watts capacity having 
a normal speed of 1,800 R. P. M. The fields were excited from storage 
battery, about 140 milamperes being used. At 1,800 R. P. M. this excita- 
tion gave about 50 volts at the terminals. Since the field was constant 
and no appreciable current was taken from the armature the voltage re- 
mained directly proportional to the speed. The condensers had a combined 
capacity of about 65 micro-farads and were of the ordinary paper type. 
The instrument used was home made and very imperfect. Its moving 
element was very heavy. its frictional error large and the damping effect 
