CHAP, x.] ELECTRICITY AND MAGNETISM. 371 



of self-induction^ and directly proportional to the resist- 

 ance of the circuit and to the time that has elapsed since 

 making circuit. 



A very brief consideration will show that in those 

 cases where the circuit is so arranged that the coefficient 

 of self-induction, L, is small as compared with the resist- 

 ance R, the fraction ^ will have a high value, and the 



R \ 



term ( jjj will vanish from the equation for all appre- 

 ciable values of /. 



Where, however, L is large as compared with R, as 

 in long coils, long lines of telegraph cable, etc., the value 

 of this term, which stands for the retardation due to self- 

 induction^ may become considerable. 



4O6. Induced Currents of Higher Orders. 

 Professor Henry discovered that the variations in the 

 strength of the secondary current could induce tertiary 

 currents in a third closed circuit, and that variations in 

 the tertiary currents might induce currents of a fourth 

 order, and so on. A single sudden primary current pro- 

 duces therefore two secondary currents (one inverse and 

 one direct), each of these produces two tertiary currents, 

 or four tertiary currents in all. But where the primary 

 current simply varies in strength in a periodic rise and 

 fall, as when a musical note is transmitted by a micro-- 

 phone or telephone (Art. 435), there will be the same 

 number of secondary and tertiary fluctuations as of 

 primary, each separate induction involving, however, a 

 retardation of a quarter of the full period. 



LESSON XXXVII. Magneto-electric and Dynamo- 

 electric Generators. 



4O7. Faraday's discovery of the induction of currents 

 in wires by moving them across a magnetic field sug- 

 gested the construction of magneto-electric machines 



