on the Magnetic Properties of Steel and Iron. 189 



somewhat larger than that of the magnets, was held firmly by 

 -corks in the axis of the coil, and the magnets pushed into posi- 

 tion by a glass rod. 



The tangent-compass was made with a single ring of copper, 

 having a mean diameter of 404 mm . 



The compass needle, being but 20 mm long, was provided with 

 a glass pointer; the scale, which was divided into degrees, was 



The order of the observations was as follows : (1.) reading of 

 the magnetometer with the circuit open; (2.) with the circuit 

 closed ; (3.) with the circuit closed and the magnet in the coil : 

 (4.) the deflection of the tangent-compass was then determined 

 ted] ; (5.) and lastly the magnetometer reading was 

 taken with the circuit open and the magnet in the coil. 



The difference between the readings (3) and (2) gives the 

 deflection due to the temporary magnetism ; that between 5 

 and 1, the deflection due to the permanent magnetism. 



14. Calculation of the Experiments. — The calculation resolves 

 itself into (a) that of the magnetic intensity in the coil, and 

 (b) that of the resulting temporary and permanent magnetism. 



(a.) Intensity of the magnetic force. — This resolves itself again 

 into two, the calculation of the intensity of the current and 

 that of the constant for the magnetic working of the coil. 



The intensity of the current was calculated by the formula : 

 i=Atg<p. The reduction factor (A=- — ) of the tangent- 

 compass to the absolute system was calculated 62 '7. 



The correction to the above formula, due to the cross-section 

 of the windings and the length of the needle lay within the 

 errors of observation, and was therefore not considered. 



(b.) Intensity of the magnetic field in the coil. 



The formula, 



?r _g=JL_ + a+x 1 



gives the magnetic effect of a coil, of the length 2a, the radius 

 r and n windings traversed by a current of intensity i, on a 

 point situated in the axis of the helix at a distance from its 



To obtain the mean intensity of the field for a bar, with the 

 length 2b, and with a cross-section so small that we may con- 

 sider the bar as lying wholly in the axis, we divide formula 

 (I) by 2/- and alter multiplying by dx, integrate between the 



(II) X«=iy X,dt=^[{{a+by + t*\i-{{a 



