Construction of Non-Inductive Resistances. 187 



resistance of the glow-lamps cannot be made simultaneously 

 with the power-test, since to do this would require the intro- 

 duction of an ammeter, and therefore of inductance, into the 

 circuit, which should be non-inductive. 



Further, carbon is unsuitable for portable resistances on 

 account of its brittle nature. Platinoid, on the other hand, 

 is flexible, has a low temperature-coefficient, and a high 

 specific resistance, though not of course nearly as high as 

 that possessed by carbon. Platinoid therefore appeared to 

 us to be the best material to employ in the construction of 

 non-inductive resistances to be used for power-tests. 



It is well known that a wire doubled on itself has a very 

 small inductance, which approximates to 3*77 times the total 

 length of the wire in centimetres as the parts approach each 

 other. This value can, as Maxwell pointed out, be reduced 

 by using flat strips instead of round wires ; and, if the strips 

 be bare and be placed vertically, it is clear that the cooling 

 action will be considerable, so that relatively strong currents 

 will produce but little rise in temperature or increase of 

 resistance. 



We therefore decided in 1887 to construct the non-inductive 

 resistance seen in fig. 1, Plate IV., consisting of twelve plati- 

 noid strips, each 6 metres long, 4 centim. wide, and 0*25 millim. 

 thick. Each strip is doubled on itself, two layers of carefully 

 shell-lacqued silk, 0-075 millim. thick, being inserted between 

 the front and back portions, The whole is bound together by 

 means of a narrow silk ribbon wrapped round spirally, con- 

 siderable gaps being left between the spires of the silk ribbon 

 so that the platinoid should have plenty of free surface for 

 cooling. 



For the purpose of expelling moisture from the silk and the 

 shell-lac varnish, when the resistance-strips are first put up, a 

 current was passed through each strip strong enough to make 

 it fairly hot. During this heating frequent short-circuitings 

 occurred from rough points of the metal piercing the silk, a 

 single layer only of which was originally employed to separate 

 the front and back portions. An additional layer of silk was 

 therefore inserted. Trouble was also experienced from the 

 rough edges of the metal causing short-circuits ; but by cutting 

 the silk wider than the metal and by folding the edge of the 

 silk over the edge of the platinoid strip, this difficulty was 

 overcome. 



The doubled strips are permanently joined up in sets of 

 three, and to the ends of each set are soldered mercury-cups 

 and binding-screws. The four sets of three can be joined up 

 in series, or in parallel, or in parallel-series by bridge-pieces 



02 



