Magnetic Permeability of various Alloys of Iron. 101 
Kelvyin’s graded galvanometers.* One great advantage of this instrument was 
the ease with which its range could be altered by sliding it to or from the solenoid 
on a grooved board, or by the addition of an over-head magnet, by means of which 
the strength of the field at the magnet could be increased by a known amount. 
The field-magnets usually employed on the instrument were too strong for our 
purpose, and we had two smaller ones specially made. The strength of these 
were found to be 0°875 and 2:°927 C.G.S. units respectively. These, with the 
Harth’s field, as carefully determined at the place of observation, viz. 0°175 
C.G.S8., gave us three convenient grades. The standard distance of the magneto- 
meter needle from the upper pole of the rod under test was 45 cms. This distance, 
in the case of a few of the less permeable steels, was reduced to either 20:9 ems. 
or 14°75 ems.; these distances being chosen because the deflections, when divided 
by 5 or 10, respectively, corresponded to those at the standard distance. This 
ratio, obtained by calculation, was verified by actual experiment at the different 
distances. 
The test-rod was supported vertically, as shown in fig. 7, inside the 
magnetising solenoid C. The latter was 120 cms. long and 1:7 cms. mean 
diameter, and wound with 2886 turns of No. 18 B.W.G. double-cotton-covered 
copper wire, in four layers. This was sufficient to carry a current of 10 ampéres 
for a short time without undue heating. The resistance of the coil was 2°15 
ohms, and the magnetising force was 30:2 C.G.S. units per ampére. 
In order to neutralize the effect on the magnetometer of the magnetic 
field due to the solenoid, a compensating coil, C’,+ was introduced into the 
circuit, so that the main current passed through it as well as the solenoid. 
By trial, on moving the compensating coil to and fro, a point of neutralization 
was found, so that, when no test-rod was present, the magnetometer needle 
remained unaffected whatever strength of current traversed the solenoid. Any 
magnetisation of the rods due to the vertical component of the Karth’s magnetic 
field was neutralized by a single layer of wire round the solenoid, through which 
a small independent current from a single cell was sent by the circuit W’. The 
strength of this current was adjusted to suit the particular rod under test by 
means of the rheostat R’, or it could be intercepted altogether by the plug key K’, 
as it was not required in the specimens of low permeability. 
The main magnetising current was obtained from 1 to 5 cells of a storage 
battery B, and after traversing the plug key K, and circuit W, entered a series of 
* This magnetic system is composed of four small magnetic needles, about 1 em. long each, with their 
poles turned in similar directions, and supported on an iridium-tipped point and sapphire cap. The 
pointer is an aluminium index, about 9 cms. long, working over a tangent scale. 
} In fig. 7 this is also marked C by mistake. The relative distances of the compensating coil and the 
magnetometer from the solenoid were much greater than would appear from the diagram. 
Q 2 
