as a Standard of Electromotive Force. 



139 



way, after measuring the potential at the ends of the 36 wires 

 in parallel, with the current passing through them, one wire is 

 disconnected, and its resistance taken immediately; from which 

 observations it is possible to approximate very accurately to 

 the resistance of the whole wires in parallel. In order to 

 afford data for constructing such resistances, experiments were 

 made of passing various currents through coiled spirals of 

 naked wires of different sizes and materials. 



A large number of wires were prepared, of copper, brass, 

 iron, German-silver, each 25 feet long, and of six sizes re- 

 spectively, Nos. 10, 12, 14, 16, 18, 20, B.W.G., the diameters 

 being given below. These wires were coiled into spirals 

 round wooden rods about one inch diameter, and the turns of 

 the wire well separated, so that each coil or spiral was about 

 18 inches long. Measured steady currents were sent through 

 these for some hours, and so adjusted that after the tempera- 

 ture had become steady the wires were all at a temperature 

 just about bearable by the hand, that is near 60° C. The 

 currents respectively carried were as follows : — 



Size of wire. 



( 



No. 10 



12 



14 



16 



18 



RW.G. 











•134 inch 



•109 



•083 



•065 



•049 



diam. 











20 

 •035 



Currents carried in Amperes. 



German Silver ... 



Brass 



Iron 



Copper 



18-75 



135 



8-25 



6 



4-12 



30 



18-75 



15 



9-75 



75 



18 



11-25 



10-5 



8-25 



5-25 



49-5 



38 



26-25 



20-25 



15 



3 



5-25 

 3-75 

 9 



These currents passed through the above-described naked 

 spirals bring the respective wires to about a temperature of 

 60° C, when equilibrium is established; and for the purposes 

 of measuring currents not more than one third of the above 

 currents should be used with wires of the size appended. 

 Thus for 300 amperes, about 50 No. 10 B.W.G. wires will 

 carry it without much sensible elevation of temperature; 

 and by arranging 50 wires so that their resistance can be 

 quickly measured in series, a resistance can be made suitable 

 for measuring the potential at the ends of a known resistance. 



By the use of this method it is easy to standardize any cur- 

 rent or potential-galvanometer at any part of the scale, and 

 obtain the absolute or corrected value of the deflections ; and 



