1908-9.] An Improved Form of Magnetometer. 187 
of the coil are mounted on an ebonite block at one end of the solenoid. 
D and C are the inlet and outlet tubes of the water-jacket. Although the 
water-jacket is somewhat narrow it is found to be effective in keeping the 
helix of wire cool, even though the interior is raised to a temperature of 
over 1000° C. by means of an electric furnace. The water-jacket is made 
small in capacity in order to keep down the mean radius of the solenoid, and 
lienee maintain the end effect of the solenoid small. The field at the centre 
of a coil of length 2 1 and radius a is less than that given by O’^irnC in the 
ratio (l 2 — 2a 2 )/l 2 , where n is the number of turns in the coil per unit length 
and C is the magnetising current in amperes. In the case of the solenoid 
now described the reduction in the field from the value 0'47mC due to the 
finite length of the coil is only 1T4 per cent. The solenoid is carried on a 
mahogany base-board provided with two vertical supports terminating in 
V-shaped grooves to receive the coil. The position of the solenoid carrier 
in the channel of the magnetometer board may be fixed by means of a brass 
clamp (shown in fig. 3). This friction clamp is furnished with two screws 
which press mahogany blocks against the outer surfaces of the wooden 
strips forming the channel of the magnetometer bed-plate. 
Cj_ and C 2 (figs. 2 and 3) are circular coils of 15 cm. radius erected on 
wooden stands provided with brass clamps as in the case of the solenoid. 
Each coil is wound in three sections, the terminals of which are screwed 
into the base of the stand. The sections in the case of C x contain 5, 7, and 
9 turns of wire respectively, and in the case of C 2 6, 8, and 10 turns. 
These sections may be used singly or in combination, and accordingly there 
is a wide range of variability in the powers of the coils. C 3 is a coil of 
similar construction, but has a radius of only 6 cm., and is built in two 
sections of 1 and 3 turns of wire respectively. 
D is a coil having a radius of 12 cm., and its function is to prevent loss 
of time due to the needle vibrating about its position of equilibrium. It is 
connected up in series with a single cell and a reversing key ; and by 
properly tapping the key a series of impulses is communicated to the 
needle, which is thus quickly brought to rest. 
L is a further sliding stand carrying the object screen. This consists of 
a vertical wire placed in front of a window of obscured glass fitted in a 
metal box containing an incandescent lamp. By altering the position of 
this stand, the image of the cross-wire formed by the mirror of the magneto- 
meter can be produced at any distance from 110 cm. upwards. From 
150 cm. to 200 cm. is in most cases a suitable value. At this distance it 
is received on an engine-divided glass scale of the usual type. 
E is a deflector stand on which a small permanent magnet may be 
