10 



PKELIMINAEY NOTE ON A MAGNETIC PEOOF PLANE 



[American Journal of Science [3], X, 14-17, 1875] 



About four years ago I made a large number of experiments on the 

 distribution of magnetism on iron and steel bars by means of a coil of 

 wire sliding along the bar; the induced current in the coil as measured 

 by a galvanometer was a measure of the number of lines of force cut by 

 the coil and can be found in absolute measure by my method of using 

 the earth inductor. These researches have never yet been published 

 owing to circumstances beyond my control, but are known to quite a 

 number of persons in this country, and will soon be published. The 

 method there used is the only correct one that I know of for experi- 

 menting on magnetic distribution, and my purpose in this note is to 

 extend it to bodies of all shapes, so that experiments on magnetic dis- 

 tribution may become as simple and easy to perform as those on elec- 

 trical distribution. And so well has my magnetic proof plane accom- 

 plished this that I can illustrate the subject to my classes with the 

 greatest ease. 



The apparatus required is merely a small coil of wire i to ^ inch in 

 diameter, containing from 10 to 50 turns, and a Thomson galvanometer. 

 When we require to reduce to absolute measure, another coil about a 

 foot in diameter and containing 20 or 30 turns is required. Having 

 attached the small coil (or, as I call it, the magnetic proof plane) to 

 the galvanometer, we have merely to lay it on the required spot, and 

 when everything is ready, to pull it away suddenly and carry it to a 

 distance, and the momentary deflection of the galvanometer needle will 

 be proportional to that component of the lines of force at that point 

 which is perpendicular to the plane of the coil. And if we apply it to 

 the surface of a permanent magnet the so-called surface density of the 

 magnetism at that point will be nearly proportional to the deflection. 

 In the case of an electro-magnet the surface density will be nearly pro- 

 portional to the deflection minus the deflection which would be pro- 

 duced by the helix alone, though the last is generally small and may be 

 neglected. I use the words nearly proportional in the above statement 

 because thev are only exactly true in the cases where the lines of force 



