223 
Almost any condenser when suddenly discharged if allowed to stand a 
few minutes will experience a rise in potential at its terminals. This rise is 
due to what is known as the residual charge. This phenomenon is explained 
as follows: When a condenser is charged its dielectric is strained and being 
non-homogeneous the strains are unequal. (By strain is meant the actual 
compression of the plates.) When discharged these strains are relieved but 
they do not decrease at the same rate, so that some parts of the dielectric 
become strained in the opposite sense and balance those parts which are 
slower in acting. The condenseer is then apparently discharged, but after 
standing a while these strains tend to diminish and usually there is a re- 
sultant strain set up. This resultant strain is due to the fact that while 
the forces were originally balanced at the end of the first discharge, yet 
the distances are unequal and in nonhomogeneous materials stress is sel- 
dom proportional to strain. 
The condenser may now be discharged again and after a time may 
show still another rise of potential. In the apparatus herein described 
this effect is entirely negligible, for the reason that the condenser is never 
charged or discharged suddenly, some few seconds being required to com- 
plete the action. 
In all condensers there is also some absorption, but with good con- 
densers used at the voltages proposed for this apparatus this effect is also 
quite negligible, and we may with certainty say that for a given voltage 
change at any part of the potential range equal quantities of electricity 
pass through the instrument. 
With an instrument giving a uniform scale therefore we have an ap- 
paratus which will show equal increments of readings for equal rates of 
change of velocity, i. e. a direct reading accelerometer. 
Fig. 12 shows the apparatus as assembled for use in railway work. 
The double commutator magneto is here shown belted directly to the car 
axle. It is obvious that the readings of the instrument are unaffected by 
grades or side tiltings of the car. 
The apparatus may be made self-recording by employing a recording 
instrument instead of an indicating one, as shown in Fig. 13. These re- 
corders may be obtained in the market and are very sensitive and reli- 
able. The record is made by placing a pen on the end of the voltmeter 
pointer, the whole being pulled down upon a sheet of paper moving at a 
uniform rate of motion by means of a small magnet whose circuit is 
