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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 

 can 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 u the high resistance at 

 R, and the second coil on the moving element of the instrument has its 

 terminals shown at T, and T 2 . These terminals are also shown in the 

 separate sketch of the instrument C. 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 



