6 THE MAGNETIC CIRCUIT [ART. 3 



flow of an incompressible substance takes place. For this reason 

 the magnetic flux is said to be solenoidal (i.e., channel-shaped). 



The familiar law of electromagnetic induction discovered by 

 Faraday is used for the definition of the unit of flux. Namely, when 

 the total magnetic disturbance or llux within a turn of wire 

 changes, an electromotive force is induced in the turn. By experi- 

 ments in a uniform field, the fact is established that the value of 

 the induced electromotive force is exactly proportional to the rate 

 of change of the flux linking with the test loop. This fact is 

 used in the definition of the unit of flux. 



With the volt and the second as the units of e.m.f. and of 

 time respectively, the corresponding unit of flux is called the 

 weber, and is defined as follows : A flux through a turn of wire 

 changes at a uniform rate of one weber per second when the e.m.f. 

 induced in the turn remains constant and equal to one volt. Such 

 a unit flux can be also properly called the volt-second, though as 

 yet neither name has been recognized by the International Electro- 

 technical Commission. The weber or the volt-second is too large 

 a unit for most practical purposes. Therefore a much smaller 

 unit, called the maxwell, 1 is used, which is equal to one one-hun- 

 dred-millionth part of the weber, or 



one maxwell = one weber XlO~ 8 . 



The lines of force in Figs. 1 and 11 can be made to represent 

 not only the direction of the field, but its magnitude as well, if they 

 be drawn at suitable distances from each other. That is, such 

 that the total number of lines passing through any part of a 

 cross-section of the ring is equal numerically to the number of 

 maxwells in the flux through the same part. With this conven- 

 tion, each line stands symbolically for one maxwell; some engi- 

 neers and physicists speak of the number of lines of force in a flux 

 when they mean maxwells. 



While the weber is too large a unit, the maxwell is too small for 

 many practical purposes. Therefore two other intermediate units 



1 The origin of the maxwell becomes clear when one remembers that the 

 volt was originally established as 10 8 electromagnetic C.G.S. unit of electro- 

 motive force. The maxwell is related to the C.G.S. unit of e.m.f. or the so- 

 called abvolt in the same way in which the weber is related to the ordinary 

 volt. In other words, when the flux within a coil varies at the rate of < ne 

 maxwell per second, one abvolt is induced in eac'i turn of the winding. 



