nexion 

 drawn. 



F«r. 



Stress on the Electrical Conductivity of Metals. 637 



the shoulder of the rod and the brass piece makes the con- 

 water-tight. Over this rod a rubber tube, D, is 

 completely enveloping the terminal and the wire 

 connected to it. The other terminal passes out through an 

 ebonite block closing the lower end of the rubber tube, and 

 is soldered to a wire coming from the brass piece A. The 

 rubber tube is filled with machine oil, in which 

 the wire is therefore completely immersed. In 

 the first experiments made with manganin wire, 

 the wire was wound on an ebonite cylinder 

 placed inside the rubber tube mentioned above. 

 It was found, however, that this introduced a 

 source of error into the results, shown by a 

 small bend near the origin in the curve re- 

 * presenting the relation between change of 

 resistance and pressure. A certain amount 

 of tension is necessarily used in winding the 

 wire on the cylinder, and this causes a small 

 increase in its resistance. On application of 

 pressure the ebonite cylinder is compressed and 

 the wire is freed from tension, causing a cor- 

 D responding alteration in the resistance. In all 

 further experiments great care was taken to 

 wind the wire loosely on a frame without the 

 least tension. The change of resistance was 

 measured by means of a Wheatstone's bridge 

 made up of rheostats without a slide- wire. 

 Owing to the smallness of the effect measured, 

 it was found better to observe the deflexions 

 of the galvanometer, keeping the resistances in 

 the arms of the bridge constant. A resistance-box having 

 loo" onm co ^ s AYas pl ace d in the arm of the bridge adjacent 

 to the wire under investigation, and the galvanometer-scale 

 was calibrated by altering the resistance in this box and 

 observing the deflexions of the galvanometer. As the re- 

 sistances in the other two arms were equal, this gives directlv 

 the change of resistance in the wire corresponding to a given 

 deflexion of the galvanometer. The sensitiveness of the 

 galvanometer was 1 . 10~ 8 ampere per division, and with 

 100 ohms in each arm of the bridge a change of *1 per cent, 

 in the resistance of the wire gave a deflexion of 200 divisions. 

 The chief difficulty in making the measurements arises from 

 the heat of compression in the cylinder. In order to obtain 

 a constant temperature, the cylinder was immersed in an iron 

 vessel filled with water kept continually agitated by means of 

 a small fan worked by an electric motor. The capacity of the 



