1887.] On the Heating Effects of Electric Currents. 



281 



to draw upon secondary cells for more than 10 amperes per negative 

 plate of the dimensions at present followed. The packed plates dis- 

 integrate and become damaged with too great an output of current. 

 Hence all my experiments were made with currents well within the 

 range of the battery. I obtained samples of wire of various metals 

 and of various diameters from 0'004 inch up to 0*040 inch. It is 

 convenient to take these measurements in thousandths of an inch 

 (mils), for all our manufacturers and electric light engineers in the 

 United States and the United Kingdom work to this gauge. The 

 conversion of the values thus obtained into the metrical and more 

 scientific system is very simple. The wire to be experimented upon 

 was clamped between two small brass binding screws fixed upon a 

 dry wooden stand. 



I pointed out in my previous paper how the cooling effects of the 

 terminals or binding screws might vitiate the results, and how neces- 

 sary it was to experiment on wires of sufficient length to prevent 

 any error occurring from this cause. I used lengths of 6 inches to 

 determine the constants for wires free from the cooling effect, but 

 lengths of \\ inch with massive terminals to determine the constants 

 for wires used in practice as " cut-outs." 



The cooling effect of the terminals very seriously affects the 

 efficiency of the cut-outs used in actual practice, and the larger the 

 fusing wire and the terminals the more serious is the error intro- 

 duced. On the other hand, the greater the lengths of wire used as a 

 fuse the greater the resistance inserted, and the efficiency of the 

 system itself may be reduced. Cut-outs, therefore, should be 

 employed sparingly and with judgment, and the fusing wire should 

 not be so short as to impair the fusing point. 



In the following tables I have tabulated the results of the numerous 

 experiments made. 



When we consider the irregularities in drawing these fine wires to 

 true cylinders, the difficulty in determining the current at the exact 

 moment of fusion, and the variation in the specific resistance of the 

 metals, I think the results must be considered very satisfactory in 

 support of the law. 



Three points of observation were taken : — 



1. The melting point of a small flake of shellac placed on the wire, 

 which may be taken at 77° C. 



2. The point of self-luminosity, 525° C. This was only determined 

 roughly in air without the dark chamber I employed previously. 



3. The fusing current. 



