58 SIR WILLIAM THOMSON ON THE 
through which water could be made to flow from the cistern B, and thus, by regu¬ 
lation of the temperature of the water in B, the part of the wire within the magnetizing 
coil could he kept at any temperature from about 10° to nearly 100° C. 
202. In order that the elongation and contractions produced in the wire by the 
applications and removals of the pulling stress might be observed, a second wire was 
hung from the same support, and kept stretched by two 28 lb. weights, hung from the 
ends of the cross-bar, E. To this wire a scale of half a millim. was vertically 
attached, in such a position that a pointer fixed to the magnetizing coil moved along 
it as the coil moved downward or upward with the application or removal of the 
weight. 
203. The electrodes of the magnetizing coil were connected with the studs 3 and 4 
of the commutator, Iv. One of the other pair of studs was connected with the zinc 
pole of a battery of my tray Daniells, the other stud with the sliding piece of a 
resistance-slide, R. This slide was designed for the purpose of allowing the battery 
strength to be raised continuously from 0 to nearly 1 cell, and from 1 cell to nearly 
2 cells, and so on. It consisted of a contact-making slider, S, movable along a bare 
copper wire connecting the two poles of the cell to be sub-divided. This wire, which 
was 64 metres in length, and had a resistance of 0'67 ohm, was stretched for convenience 
alternately from one side to the other of a large board, in the manner represented in 
the diagram. Thus, with the number of cells and arrangement of connexions figured 
in the diagram, when the slider was brought up as nearly as possible to C, the current 
flowing was very nearly that due to 3 cells, and when the slider made contact at any 
other point of the wire, the current flowing through the magnetizing coil was less than 
that due to 3 cells by an amount depending on the distance along the wire of the 
slider from C. 
204. A galvanometer, the resistance of which was only a very small fraction of an 
ohm, was used to measure the strength of the magnetizing current, and was so placed 
in the circuit that the current always flowed through it in the same direction : thus 
the whole range of the galvanometer scale was available for measuring the deflections 
produced by the stronger currents used, without the necessity for shifting the zero of 
the scale by means of a magnet. In these experiments at first the battery galvano¬ 
meter was placed for convenience in the circuit between the commutator K and the 
coils F, and a reversing key used to keep the current through it always in the same 
direction, but it was afterwards transferred to the position in the circuit shown in the 
diagram and the reversing key dispensed with. The deflections of the needle of this 
galvanometer were read on a scale of half millimetres placed at a distance of 75 
centims. from the mirror, and were used as the values of the magnetizing forces for 
the abscissas of the curves below. The total resistance in the circuit of the magne¬ 
tizing coil, which was measured from time to time in the course of the experiments, 
and showed little or no variation, was 3 - 828 ohms. The induction coil was placed in 
circuit with an astatic galvanometer (described in Part VI., §181, and called in that 
