96 MESSRS. J. HOPKINSON AND HE. WILSON ON THE 
diameters of 2°6, 5°16, and 7°75 centims. respectively, for the purpose of inserting 
copper coils the ends of which are brought out by means of the radial slot shown in 
fig. 2. When the pole-pieces are brought into contact as shown in fig, 1, we have 
thus three exploring coils within the mass and a fourth was wound on the circular 
portion outside. These exploring coils are numbered 1, 2, 3, 4 respectively, starting 
with the coil of least diameter. 
Fig. 3 gives a diagram of the apparatus and connections, in which A is a reversing 
switch for the purpose of reversing a current given by ten storage cells through the 
magnet windings in series; B is a Thomson graded galvanometer for measuring 
current ; and C is a non-inductive resistance of about 16 ohms placed across the 
magnet coils for the purpose of diminishing the violence of the change on reversal. 
The maximum current given by the battery was 1:2 amperes. A D’Arsonyal gal- 
vanometer of Professor AyRton’s type, D, of 320 ohms resistance; a resistance 
box E; and a key F were placed in circuit with any one of the exploring 
coils 1, 2, 3, 4, for the purpose of observing the electromotive force of that circuit. 
The method of experiment was as follows :—The current round the magnet limbs was 
suddenly reversed and readings on the D’Arsonval galvanometer were taken on each 
coil at known epochs after the reversal. The results are shown in fig. 4, in which 
the ordinates are the electromotive forces in C.G.S. units and the abscissee are in 
seconds. 
The portion of these curves up to two seconds was obtained by means of a ballistic 
galvanometer having a periodic time of fifty seconds, the key of its circuit being 
broken at known epochs after reversal. From the induction curve so obtained the 
electromotive force was found by differentiation. 
The curve A which is superposed on curve 4 of fig. 4 gives the current round the 
magnet in the magnetizing coils. It is worth noting, that, as would be expected, it 
agrees with the curve 4. The potential of the battery was 1:2 amperes xX 16°3 ohms 
= 19°6 volts. Take the points two seconds after reversal, the electromotive force in 
one coil is 330,000 ; multiplying this by 5208, the number of coils on the magnet, we 
have in absolute units 1,718,640,000 as the electromotive force on the coil due to 
electromagnetic change, or, say, 17'2 volts. Subtracting this from 19°6 we have 2:4. 
The electromotive force observed is 125 X 16°3 = 2°02. The difference between 
these could be fully accounted for by an error of + second in the time of either 
observation. 
The general character of the results was quite unexpected by us. Take coil No. 2 
for example, the spot of light, on reversing the current in the magnet winding, would 
at once spring off to a considerable deflection, the deflection would presently diminish, 
attaining a minimum after about 6 seconds; the deflection would then again increase 
and attain a maximum greater than the first after 8 seconds, it would then diminish 
and rapidly die away. 
To attempt a thorough explanation of the peculiarities of these curves would mean 
