694 
SIR WILLIAM THOMSON ON THE 
a piece of thin sheet copper, and on the outside of that there is coiled, in two layers, 
719-7 centimetres of silk-covered copper wire, the copper weighing 2-502 grammes per 
metre. The inner layer contains 326 turns and the outer 321. The resistance of this 
coil when cool is *511 ohm. Its ends are put in communication by thick electrodes 
with a reversing-key, k, and a battery of three of my “ tray ” Daniells. The resistance 
of each of these cells is about -06 of an ohm, giving for the whole battery a resistance 
of -18 ohm. 
180. Over the coil C C' is wound another coil DD' 9-8 centimetres long, which con- 
tains 538 centimetres of wire, No. 26 of the Birmingham wire-gauge, with 38 centi- 
metres for electrodes. This wire is also wound on in two layers, the inner containing 
147 turns and the outer 146. The resistance of this induction-coil is T432 ohm, and 
its weight per metre 1-189 gramme. 
181. The deflections of the galvanometer are read in my usual manner by the image of a 
fine wire fixed vertically close in front of the edge of a flat paraffine- or gas-flame. The 
screen on which the image is thrown is a white paper scale, divided into fortieths of an 
