264 Prof. J. A. Fleming on a Method of determining 



The method is based upon an interesting discovery with 

 regard to the distribution of the induction in a bundle of 

 straight iron strips or iron or steel wires, when placed in a 

 uniform magnetic field. 



If a long, straight, magnetizing coil is prepared by winding 

 insulated wire upon a paper or vulcanized fibre tube, and if 

 the length of the solenoid is, say, ten times its mean diameter, 

 then for a distance equal to at least three-quarters of the total 

 length of the coil we have a practically uniform magnetic 

 field in the interior of the bobbin, when a constant current is 

 sent through the wire. If into this uniform magnetic field a 

 bar of iron or straight bundle of iron strips or iron wire is 

 introduced, the length of the iron being not greater than the 

 uniform portion of the interior field of the coil when the iron 

 is not there, this iron has magnetic induction created in it 

 which varies in density from point to point in the bar, but is 

 symmetrical in value about the central point of the bar. 



The induction density (B) is greatest in the centre of the 

 bar but falls off towards each end. If the magnetizing coil is 



V w 



C. Magnetizing coil. 



V. Electrostatic voltmeter. 



Rb. Rheostat. 



I. Iron strips. 

 W. Wattmeter. 

 A. Alternator. 



v. Exploring coil. 

 D. Dynamometer. 



traversed by an alternating current, and if the iron is 

 laminated, the maximum value of the induction density in 

 the bar can be easily determined at any point in its length by 

 placing upon the bar a flat bobbin of fine insulated wire which 

 is connected with an electrostatic voltmeter capable of reading 

 from 1 to 20 volts (see diagram). The first portion of this 



