ELEMENTARY THEORY OF ELECTROMAGNETISM. 51 



where E is the propelling 

 force, L is the mass of the 



boat, and jr is the rate of in- 



crease of the velocity. If 



dl 



dt 



is negative it signifies decreas- 

 ing velocity. 



where E is the electromotive 

 force acting on the circuit (of 

 zero resistance), L is the in- 



dl 



ductance of the circuit, and 37 



at 



is the rate of increase of the 



. dl . 



current. If ~r. is negative it 

 01 



signifies decreasing current. 



The units of force, mass and acceleration which are used in 

 equation (i) are familiar to the student of elementary mechanics. 



When the rate of increase of current in equation (2) is expressed 

 in abamperes per second and E in abvolts, then L is expressed in 

 abhenrys. That is to say, a circuit has an inductance of one 

 abhenry when an electromotive force of one abvolt (in excess of 

 the electromotive force RI required to overcome the resistance 

 of the circuit) will cause the current in the circuit to increase at 

 the rate of one abampere per second. 



When the rate of increase of current is expressed in amperes per 

 second and E in volts, then L is expressed in Henrys. - That is 

 to say, a circuit has an inductance of one henry when an electro- 

 motive force of one volt (in excess of the electromotive force RI 

 required to overcome the resistance of the circuit) will cause the 

 current in the circuit to increase at the rate of one ampere per 

 second. One henry is equal to io 9 abhenrys. 



34. Further discussion of self -induced electromotive force. 



According to Art. 32 an electromotive force equal to L is 

 induced in a circuit by a changing current, and according to Art. 

 33 an electromotive force equal to -f- L must act on a circuit 



(in addition to the electromotive force RI required to overcome 

 resistance) to cause the current to increase. This matter may be 

 made clear as follows: 



