THE EARTH’S MAGNETIC FIELD IN INTERNATIONAL UNITS. 
437 
grooves were turned. That the edges of the grooves lay accurately in one plane 
was shown by clamping a microsco23e to the lathe and rotating the coils while 
the cross wire of the microscope was adjusted on the edge of the groove. In tliis 
way it was found that the sides of the grooves nowhere departed from a plane 
by 0'002 centim. 
Small ebonite bushes were inserted in the Mange where the wires leading to the 
coils had to pass. The wire employed was number 34 S.W.G., the uncovered wire 
having a diameter of 0'023 centim. This wire was covered with three coatings 
of white silk. In order to allow of a test of the insulation being made after the 
coils were wound, the wire was wound double so that each coil consisted of two 
independent circuits, the wires of which lay alongside each other throughout their 
whole length. Hence by testing the insulation between these two circuits, and 
between each circuit and the flanges, a satisfactory test of the insulation of the wire 
could be made. These tests, however, would not allow of the detection of the 
short-circuiting of one or more turns of either of the circuits. This point could be 
tested by sending the same current through the two circuits in opposite directions. 
The absence of any magnetic field at the centre of the coils showed that the 
magnetic effect of each of the circuits was the same, and hence, as it is very unlikely 
that exactly the same number of turns of each circuit could be short-circuited 
without a connection Ijeing formed between one circuit and the other, these tests 
may be taken as sufficient. The width of the grooves was made such that twelve 
turns of wire, that is six turns of eacli circuit, just fitted in. There were eight 
layers in each coil, and since the two circuits were always used in parallel this 
gave 48 turns in each coil. After winding each layer it was given a coating of weak 
shellac varnish, made by dissolving shellac in absolute alcohol. This varnish was 
soaked up by the silk covering of the wire and served the purpose of preventing 
the silk taking up moisture from the air and thus losing in part its insulating 
properties. The wires joining the two coils were led along the cylindrical surface of 
the coils in the same horizontal plain as the centre of the coils. In this position 
these wires produce at the centre of the coils no magnetic field which has a 
horizontal component, and hence the fact that they are distant from one another by 
about 1 millim. from centre to centre can produce ]m error. 
The insulation of the circuits was tested before and after the experiments and was 
found to exceed 200 megohms both from one circuit to the other and from each of 
the circuits to the flanges. The insulation of the rubber-covered leads and of 
the Pohl commutator used to reverse the current was also tested and found 
practically infinite. 
The metal of which the flanges were cast was specially mixed by the founders so 
as to avoid the presence of any iron, and these castings, as well as all metal used in 
the construction of the coils, were carefully tested for magnetism by means of a 
delicate magnetometer. The fact that the instrument is free from magnetism was 
