Theory of Magnetism. 249 
induction-coil. The iron plates were held fast together by 
thin brass bolts which traversed the axes of the solenoids. The 
two solenoids and the two iron plates formed together a closed 
horseshoe-magnet, whose magnetic moment was to be mea- 
sured by means of the induction-coil on the iron plates. In 
the following Table VI. these measures are brought together. 
It will be seen from the table that all the phenomena were 
exactly the same as if the iron plates had been connected by 
iron cylinders instead of brass bolts. On actually making the 
experiment with iron cylinders of 4 millim. thickness (which 
had therefore =}, of the section of the solenoid*), the magnetic 
moment was, in fact, nearly twice as great as before with a 
current-strength 0:2 ampere, as shown in the sixth column of 
the table, which gives the quotients of the deflections with 
and without iron cylinders. That with feeble currents the 
quotients are not greatly different from unity, but then rise 
rapidly to the double value, and then slowly decrease, is due 
to the property possessed by the keepers of this air-magnet, 
and the iron cylinder of opposing a great resistance to mag- 
netization, both with very feeble magnetic moments and 
with very powerful ones. 
TaBLE VI. 
. | As closed | Closed and Ratio of 
Inten-| Solenoid { As open horseshoe. with iron bolts.) deflections | 
sity. alone. | horseshoe. Mi B cenee 
Defi. Incr. | Defl. Incr. | Defi. Incr. | Defi. Incr. 
0-01 Sy <5 Bye ar be & 65 65 1:18 
0:05 270 855 295 61 380 79 1-27 
Pere oe. 52 580 ~=62 640 68 | 1000 124 1:56 
0-15 900 64 | 1020 76 | 1920 184 1:88 
0-20 1236 67 13892 74 2864 189 2-06 
0°30 $928) 018.0) 2160. | 77 4480 162 2:07 
0°40 2616 69 | 2960 80 5850 31387 1:97 
W507) 258 8 85°2° | 3360. 76 1 3800 84 7200 185 1:89 
0°75 ZION) | GOTS. OO! 10250 122 1:68 
1:00 7240 78 | 8400 93° | 12880 105 1:53 
As a direct proof of the correctness of the assumption that 
in an iron bar round which electric currents circulate only so 
much magnetism can be produced as is connected with the 
sum of the magnetic moments of the air or space in contact 
with its surface, the following considerations may be men- 
* If we take the diameter of the active air-cylinder as extending to 
the middle of the coils, then the ratio of sectional areas aban 2G 
air 9560 
