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148 
close to the wood drum on which the wire was coiled and 
the temperature noted at the time of each experiment. 
The circuit was first closed by pressing a button and then 
connection with the galvanometer was made. By a suitable 
arrangement of keys the current was allowed to circulate 
through the wire such a short time that it could not sen- 
sibly increase its temperature. 
The wire was tested for breaking-strain in an apparatus 
similar in principle to a steel-yard weighing macnine, the 
weights could be added without any jar and the elongation 
recorded at the moment of rupture. The length tested 
measured always 10 inches between the jaws of the machine. 
The diameter was measured by a decimal guage, manu- 
factured by Elliott Bros., reading to i oVo inch. 
The tortion tests were made by gripping a wire in two 
vices 8 inches apart and turning one of them on to a 
suitable apparatus till the wire broke. 
The results are given in table A. Charcoal iron is here 
seen to have the least electrical resistance, or about one half 
that of piano steel. We also notice a regular increase of 
resistance as the impurities augment. Charcoal iron is al- 
most chemically pure, while dephosphorized iron, the next 
on the list, has some impurities which increase in ordinary 
iron v/ire. Again in the samples of steel, piano steel wire, 
which has the highest electrical resistance, contains two or 
three times as much carbon as mild steel, while steel wire 
samples 3(X and 2a are intermediate in carbon as they are 
intermediate in electrical resistance. 
Table A shows that the breaking strain and electrical 
resistance increase together. 
A series of eight sets of experiments were made on 
various specimens of charcoal iron wire to determine the 
effect of annealing on the electrical resistance, but without 
definite result, as the alteration caused by annealing appears 
