8G HENKY A. KOWLAND 



proceed from the surface in a perpendicular direction; otherwise the 

 deflections must be multiplied by the secant of the angle made by the 

 lines of force with the surface of the magnet. In the case of an electro- 

 magnet made of very soft iron, theory shows that the lines pass out 

 nearly perpendicular to the surface and so no correction is needed. 



We can also, by a coil of this kind, determine the intensity of the 

 magnetic field at any point and thus be able to make a complete map 

 of it. Having done this, we have all the data necessary to substitute 

 in the formula which I have given in this Journal, 1 and by a simple 

 experiment can thus determine the coefficient of magnetization of any 

 diamagnetic or weak paramagnetic body probably in a more accurate 

 manner than any Weber used. Only the largest-sized magnets could of 

 course be used for this purpose with any accuracy, and indeed they are 

 always to be preferred in obtaining the distribution by this method. 



Having obtained the distribution for any given magnet, the distribu- 

 tion for any similar magnet of the same material but of different size 

 becomes known by a well-known law of Sir William Thomson. 



As, in the present state of our knowledge, magnetic measurements 

 are of small value unless made on the absolute scale, we require to 

 reduce our results to this system. There are several methods of doing 

 this, but the simplest is that which I have used in my experiments on 

 magnetic permeability, and consists in including an earth inductor in 

 the circuit. A coil laid on a perfectly level surface is sufficient for 

 this : when this is turned over, the induced current will be equal to C = 



%n ~VA 



where n is the number of turns in the coil, A its mean area, V 

 -Ti- 

 the vertical component of the earth's magnetism, and R the resistance 

 of the circuit. When the small coil is pulled suddenly away the current 



will be C" = *-&?, and so we have Q = 2V^, in which when a 

 li an 6 



Thomson galvanometer is used C' and C can be replaced by the cor- 

 responding deflections: hence = 2V~-, in which a and n' are the 



an D 



area and number of turns in the small coil and Q is that component of 

 the magnetic field we are measuring in the direction of the axis of the 

 small coil. 



As an illustration of this method I will give a few experiments made 

 with the magnets of a Euhmkorff diamagnetic apparatus, which was 

 altogether about 2 ft. long and had its magnets 2 in. in diameter, with 



'On a new diamagnetic attachment to the lantern, &c., this Journal, May, 1875. 



