180 MR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 
structed, consisting of two independent coils, each containing eighteen feet of 
silked copper wire. These coils were exactly alike in shape and number of 
turns, and were fixed side by side with a small interval between them, in which 
a double needle could be hung by a fibre of silk exactly as in the former in- 
strument (8 7 -)‘ The coils may be distinguished by the letters K L, and 
when electrical currents were sent through them in the same direction, acted 
upon the needle with the sum of their powers ; when in opposite directions, 
with the difference of their powers. 
206. The compound helix (199. 8.) was now connected, the ends A and B 
of the iron with A and B ends of galvanometer coil K, and the ends A and B 
of the copper with B and A ends of galvanometer coil L, so that the currents 
excited in the two helices should pass in opposite directions through the coils 
Iv and L. On introducing a small cylinder magnet within the helices, the gal- 
vanometer needle was powerfully deflected. On disuniting the iron helix, the 
magnet caused with the copper helix alone still stronger deflection in the same 
direction. On reuniting the iron helix, and unconnecting the copper helix, the 
magnet caused a moderate deflection in the contrary direction. Thus it was 
evident that the electric current induced by a magnet in a copper wire was far 
more powerful than the current induced by the same magnet in an equal iron 
wire. 
207- To prevent any error that might arise from the greater influence, from 
vicinity or other circumstances, of one coil on the needle beyond that of the 
other, the iron and copper terminations were changed relative to the galva- 
nometer coils K L, so that the one which before carried the current from the 
copper now conveyed that from the iron, and vice versa. But the same striking 
superiority of the copper was manifested as before. This precaution was taken 
in the rest of the experiments with other metals to be described. 
208. I then had wires of iron, zinc, copper, tin, and lead, drawn to the same 
diameter (very nearly one twentieth of an inch), and I compared exactly equal 
lengths, namely sixteen feet, of each in pairs in the following manner: The 
ends of the copper wire were connected with the ends A and B of galvanometer 
coil K, and the ends of the zinc wire with the terminations A and B of the 
galvanometer coil L. The middle part of each wire was then coiled six times 
round a cylinder of soft iron covered with paper, long enough to connect the 
