466 



Mr. W. H. Preece. On the 



[Apr. 3, 



in j are it. Hence a protecting wire has been added, which, while it 

 will allow all working currents to pass without retardation, will not 

 allow a current strong enough to damage the cable to flow through 

 without its being fused. It thus acts as a safety valve. The length 

 and dimensions of this wire have not been hitherto determined by any 

 stringent conditions. After having determined the conditions re- 

 quired to give the maximum effect with the so-called " plate pro- 

 tector "* I became anxious to find out the conditions that would give 

 a similar result with the " wire protector." 



Now, the very strongest current that can, under any possible 

 circumstances, enter a wire from the working batteries is one of 

 500 milliamperes ("5 B.A. unit current), while atmospheric currents 

 enter the wires of all strengths from 1 milliampere to 30 or 40 

 amperes, or even more. Therefore, it was desirable to find that wire 

 which would not fuse with 500 milliamperes, but would fuse with 

 currents of about 700 milliamperes and over. 



Hence, I took a source of electricity, which was a large thermopile 

 of Clamond's make, which would give the necessary current, a 

 rheostat by which it was possible to regulate this current to any 

 strength, a Thomson mirror galvanometer which enabled me to 

 measure, record, and regulate every change of current, and a dis- 

 charger by which I was able to hold, measure, and adjust the wires 

 to be tested. 



1. With a given current and a uniform wire of given diameter and 

 given material, the effect, allowing for the cooling effect of the 

 binding screws, was independent of the length, for whether 

 the length experimented upon was an inch, a foot, or a yard, it 

 always fused when the current reached a certain definite strength. 

 The point of fusion was irregular, for it simply depended on some 

 irregularity in the uniformity of the wire, and the weakest link in 

 the chain went. 



2. Samples of platinum wire of various diameters, each of 6 inches 

 in length, were taken, and the current increased gradually until the 

 wire fused. The results are given in the following table : — 



Diameter in parts of 

 an inch. 



Fusing current in amperes. 



•00050 



•277 



•00075 



•356 



•001 



•437 



•002 



•790 



•003 



1-150 



* British Association, 1880. 



