658 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
Copper conductor. Iron conductor, 
inches. inches. 
Whole length ... 16 24 
Breadth 1 2 
Depth 2 ^ 3 
Depth of hollows . . 2 2 ^ 
Diameter of hollows. 4 -r 
These relative dimensions were chosen so that the conducting powers of the two 
bars for electric currents, and consequently for heat also, might be not very unequal. 
A vessel of tin-plate, perforated to admit the bar through its sides, was soldered 
round the middle of each conductor, and two others so as to leave about 2 inches at the 
ends of the conductor projecting beyond them. The parts of the conductors within 
these vessels were about 3 inches long in the copper and 4^ inches in the iron, and the 
parts between the middle vessel and the vessels at the two sides were and 2 inches 
respectively. The bores for the thermometer bulbs were exactly in the middle of the 
last-mentioned parts. In experimenting on either conductor, the central vessel was 
generally filled with oil or water, and kept hot by a gas-lamp below it. Streams of cold 
water from the town supply-pipes were kept flowing through the two lateral vessels. 
22. To make these streams constant, whatever variations of pressure might occur 
in the supply-pipes, a cistern in a fixed position above the conductor was kept full 
(overflowing), and the coolers were supplied by pipes from this cistern. The supply 
often failed for several minutes, and sometimes for much longer ; and after an expe- 
riment (Nov. 19, 1853) was nearly lost from this cause, a plan was arranged to 
lift water up from a larger cistern (into which the exit-streams from the coolers were 
discharged), and to pour it into the smaller cistern above, so as to keep the stream 
constant in quantity (although not quite invariable in temperature) even when the 
proper supply failed. 
23. A galvanic battery for exciting a current through these conductors was pre- 
pared, consisting at first of four, and ultimately of eight, large iron cells, each 
measuring internally 12 inches deep, 10^ inches broad, and 2^ inches from side to 
side; eight porous cells, each 12 inches deep, 10 inches broad, and 2 inches from 
side to side; and eight zinc plates, each 9^ inches by 10 inches. The iron cells 
were charged with a mixture of nitric acid two parts (bulk), sulphuric acid three 
parts, and water two. The porous cells were charged with dilute sulphuric acid. 
In each of the cells there were 1^ square feet of zinc surface exposed to 2^ square 
feet of iron, and the electro-motive force was not far from double that of a single 
cell of Daniell’s. 
24. After preliminary experiments in which, with oil in the central vessel kept hot 
by a gas-lamp, the temperatures were too unsteady to allow any results of value to 
be obtained, water was substituted for oil in the central vessel, and was kept boiling 
briskly by the gas, the place of the water evaporated being frequently supplied by 
small quantities of boiling water poured in, so that ebullition never ceased. The 
