212 



meter (with the extra resistance removed) and "A" is a direct current 

 magneto generator having permanent magnet fields. 



The equation of the condenser is Q=EC ; where Q is the quantity of 

 electricity in Coulombs (ampere seconds), E is the voltage impressed, and 

 C is the capacity in farads of the condenser. Studying this equation we 

 find that if E is increased uniformly the quantity of charge Q on the con- 

 denser plates will also increase uniformly. Since Q is increasing uni- 

 formly with respect to time, the inflow of current is at a constant rate. 



i e i = t^- Likewise a constantly decreasing E will give a constant 

 '' dt 



outflow of current. However, as soon as E reaches a fixed value all cur- 

 rent flow in the circuit ceases since it is one property of the electric con- 

 denser to arrest the flow of direct current. (The terms "inflow" and 

 "outflow" refer tc those condenser plates that are directly connected ro 

 the instrument terminal. Of course as much current flows on to one set 

 of plates as flows off of the other plates, the current in the line having a 

 definite direction during an increase of voltage and the opposite direction 

 during a decrease of voltage.) The magnitude of these currents are shown 

 by the direct current instrument which consists merely of a coil swinging 

 in a uniform magnetic field. So long then as the voltage is changing uni- 

 formly the instrument will read a constant value returning to zero only 

 when E ceases changing. It follows that if E does not change uniformly 

 the instrument will not read a constant value but that its indications 

 will be proportional to the instantaneous rate of change of the voltage. 

 The direct current magneto is so designed that its voltage is directly pro- 

 portional to its speed, so that changes of voltage at its terminals can only 

 occur as a result of changes in speed. Therefore the instrument reads 

 the rate of change of speed, i. e. acceleration whether positive or negative. 

 In a preceding paragraph it was implied that an electric condenser 

 allows no current to pass when the voltage E has reached a fixed value. 

 This would be a fact if an ideal condenser could be made, but it is a well 

 known fact that there is always some leakage even in the best condensers. 

 This means that the dielectric has a definite value of resistance which 

 varies with different conditions and substances, and according to Ohm's 



"p 

 law the leakage current will be 1 = — . This state of affairs renders our 



K 



ideal circuit incorrect for any speed above zero because the instrument 

 gets a small current in a definite direction that is practically proportional 



