472 E. L. Nichols — Electrical Resistance of the 



The materials used in Mr. Blood's investigation were the 

 pure copper of commerce and " ferro-manganese " containing 

 seventy-nine per cent of manganese. These were fused to- 

 gether in a small carbon crucible, the sides of which were 

 lined with porcelain. The mixture was placed in the bottom 

 of the crucible in contact with the carbon, which was con- 

 nected with the positive pole of a storage battery. A carbon 

 pencil, connected with the other pole of the battery, was then 

 Drought into electrical contact with the charge from above, and 

 quickly withdrawn to a distance of a few millimeters. Under 

 the action of the arc thus formed, a charge weighing three or 

 four grams was completely fused in about thirty seconds, 

 without any considerable oxidation of the metals. The irreg- 

 ular ingot thus formed was afterward rendered more nearly 

 homogeneous by being re-melted in the voltaic arc, for which 

 purpose it was removed from the crucible and placed upon a 

 metal plate. The globule of metal thus obtained was rolled 

 into a thin strip, from which was cut a piece fifteen centime- 

 ters long and of such width as to give a resistance of about 

 eight-tenths of an ohm. 



The method of measurement employed was as follows : 

 The strip of alloy to be tested was connected in series with a 

 comparison standard, the resistance of the two being approx- 

 imately equal. These, together with a third resistance of about 

 one hundred and fifty ohms, formed the outer closed circuit of 

 a single gravity cell. " Potential wires " from the terminals of 

 the strip of alloy and of the standard, were joined to a switch 

 of such construction that a mirror galvanometer could be con- 

 nected in shunt with either. The galvanometer had a resis- 

 tance of two thousand ohms. A comparison of deflections 

 when the galvanometer was shunted around the resistance 

 standard, and around the test piece, afforded data for the cal- 

 culation of the resistance of the latter. 



This method is exceedingly sensitive, and when properly 

 conducted, it is capable of a high degree of accuracy. In the 

 experiments under consideration, a check upon errors arising 

 from fluctuations in the amount of current traversing the test 

 piece and from changes in the constant of the galvanometer, 

 was obtained by ever repeated reference to the indications* of 

 the latter when connected with the terminals of the standard. 



In order to relieve the observer of the necessity of main- 

 taining the reference standard at a constant temperature, or of 

 applying temperature-corrections to the results obtained, a 

 compensated carbon standard, of the type recently described 

 by the writer, was constructed.* The resistance of this stand- 



* On Compensated Resistance Standards; Transactions of the American In- 

 stitute of Electrical Engineers, vol. v, No. 10, 1888. 



