300 
Proceedings of the Boyal Society of Edinburgh. [Sess. 
nected in series so that they mutually oppose each other, measure the 
longitudinal induction in the shell of the steel tube only, so long as the 
field in the air space may be regarded as of equal intensity within and 
without the steel tube. A departure from this condition may occur when, 
for instance, the yoke is highly saturated, but even then the error intro- 
duced will be small. 
The second exploring coil of 600 turns is too sensitive to advantageously 
measure the longitudinal induction when produced by the longitudinal 
field (solenoid). It is, however, essential for the main purpose of this 
investigation, and it measures, after the iron has been demagnetised by the 
spiral field, the longitudinal induction B L (now the transverse component), 
when the circular field H c due to the central wires is increased by incre- 
ments from zero. Under these circumstances -no longitudinal field exists 
in the area enclosed by the shell of the steel tube, and no compensating coil 
is required. 
The magnetising solenoid consists of two layers of copper strip B B of 
rectangular section wound in six lengths, their ends dipping into mercury 
cups as shown. These ends may be connected in series, three series 
two parallel, two series three parallel, or all in parallel. The strip was 
of a width sufficient to permit a close winding of 3*68 turns per cm. of 
lengths of both layers. In this way it was arranged that the solenoid 
and 36 of the central wires along its axis produced in the shell of the 
steel tube the same value of longitudinal (H L ) and circular (H c ) fields 
per ampere respectively. 
The circular field is produced by the closely packed straight copper wires 
lying lengthwise within the steel tube and symmetrical about its axis. 
The ends of these wires dip into a series of mercury cups, three wires only 
being shown in the diagram. They can be connected up in any way, but 
for obvious reasons the combinations are limited to those symmetrical about 
the axis of the steel tube. 
When 36 of the central wires and solenoid are in series, the iron can be 
demagnetised by a spiral field at an angle 6 = 45° with that (H c ) subsequently 
applied, the two-way switch S having cut the solenoid out of the circuit. 
A sufficiently large number of other combinations can be obtained to 
demagnetise the steel tube at various angles. A commutator, reversing the 
current in the central wires relative to the solenoid, changes a right-handed 
into a left-handed spiral force in the shell of the steel tube. 
The terminals TT are in circuit with 16 secondary cells, a Weston 
milliampere meter with shunts, resistances, commutators, revolving 
rheostat, and revolving current reverser, so that the various operations 
