172 ROYAL SOCIETY OF CANADA 



was tightly screwed to a heavy oak beam, and through it protruded t|wo 

 studs witih threaded ends; the other plate iitted exactly over this one. 

 the two being secured together by means of bolts on the studs. The wire 

 hanging from the plates was about ten feet long, and carried at the bo^ 

 torn a scale-pan for holding the weights which loaded the wire. 



The current was led to the wire by a heavy copper lead firmly 

 soldered to the suspension plates, and led away by a light 

 flexible copper wire solderied to the juncture of the test wire and the pan. 

 In series with the wire, so that the same current passed through iij, was a 

 standard resistance. Light potential leads were neatly soldered to the 

 te&t wire al^ points a measured distance apart; these led to mercury cups, 

 from which they could be connected to the galvanometer and potentio- 

 meter. Fotential wires from the terminals of the stiandard resistance, 

 and also from a standard cadmium cell, led to other mercury cups to be 

 connected at will with the galvanometer and potientiometer. (The pur- 

 pose of the standard cell was to test the steadiness of the current.) 



The source of current was a battery of four storage cells, of 200 

 ampere-hour capacity each, which could be connected for 2, 4, 6 or 8 

 volts as required. The ciirrent from this battery is remarkably steady, 

 and by means of a very low resistance rheostat in the circuit the current 

 was made to vary less than a few parts in 10,000. The current was 

 never large enough to appreciably raise the temperature of the wire. 



The stiandard resistance was one which has been used in various in- 

 vestigations carried out by Dr. Barnes. A description of it is given in 

 his paper " On the capacity for Heat of Water between the Freezing and 

 Boiling Fomts." (Thil. Trans., Series A, Vol. 199, 1903). The stan- 

 dard consists of two coils of resistances 



j 1.00175— . 000250 (« — 2G°C.) l and 

 I 1.00043 — . 000246 (f — 20°(7.) I ohms. 



These can be placed in parallel or in series. 



The coils were kept at a constant temperature by keeping them im- 

 mersed in a paraffin oil bath, which had in it a coil of metal tube through 

 which a steady liow of cold watier passed, the oil being constantly stirred 

 by means of a small faû-screw revolved by a small water-motor. The 

 supply of cold water was taken from a constant water head, arranged OY 

 running the water from a . tap into a vessel with outflow and over- 

 flow tubes. 



A thermometer in the oil bath gave the variations in the tempera- 

 ture of the coils. The Thompson- Varley slide that was used is the one 

 used in the Ma'cdonald Physics Laboratory for accurate potential work; 



