MESSRS. W. R. BOUSFIELD AND W. ERIC BOUSFIELD 



by step from 0'5 to 5 amperes, and the resistances measured thereby. Special 

 insulation precautions were taken. The oil-bath of the manganiu resistance was kept 

 constantly stirred, and the temperature of the bath during the experiment only rose 

 from 17'9 C. at the start to 20'l C. at the end, the change of resistance due to this 

 slight change of bath temperature being for our present purpose negligible. 



In curve D the results of this experiment are set out, the ordinates representing 

 llBMtlllinm as before, but the abscissa; now representing amperes. It will be noted 

 that the form of the resistance-current curve closely follows that of the resistance- 

 temperature curve, and it might be supposed for a moment that the resistance might 

 be taken as an index of the actual internal temperature of the wire when the current 

 is passing, as in platinum thermometry. Thus when 4 '4 amperes are flowing the 

 resistance is that corresponding to a temperature of 80 C. on curve A. It is, 

 however, clear on consideration that the temperature of the wire could not be 60 C. 

 above the temperature of the bath. With a mercury thermometer-resistance in a 

 glass tube of about 1 mm. internal diameter, the rise of the mercury on passing a 

 current of 5 amperes indicates a temperature only of five or six degrees above the 

 bath, although the heat generated has all to escape through the glass walls of the 

 tube. An increase of temperature of 60 C. in the case of the 1 '2 mm. wire with 

 4 to 5 amperes was clearly out of the question, and we think that such increase, could 

 not have exceeded 2 C. or 3 C. with a cooling surface of 16 cm. 2 per watt. We 

 infer, therefore, that the passage of a heavy current through an alloy such as 

 manganin produces an effect on the molecular structure of the wire whilst the current 

 is passing, which affects the resistance in the same way as heating the wire would do, 

 but to a much greater extent. Thus the passage of a current of 4 '4 amperes through 

 our manganin wire has about the same effect on the resistance as would be produced 

 by raising the temperature of the bath to 80 C. We propose to designate this effect 

 of the passage of a current through an alloy as a thermoid effect, since, although it is 

 not a thermal effect depending on increase of temperature of the wire, the resulting 

 change in the resistance appears to be of the same kind as that produced by heating 

 the wire. The ordinary hysteresis effect we regard as a comparatively small residuum 

 from the thermoid effect. It would naturally follow that if the passage of the current 

 produced a molecular strain there would be a residual effect after the cesser of the 

 current, though the greater part of the thermoid effect mignt quickly disappear. 



Die above test was founded on the assumption that there was no thermoid effect 

 in the mercury thermometer-resistance, so that the scale readings would give accurate 

 indications of the resistance even when a heavy current was passing. A priori this 

 s probably true, since a hysteresis effect indicates a state of strain which is gradually 

 disappearing, and no hysteresis effect is observable with a resistance of liquid 

 mercury. 



It is not an easy matter to devise a test. If a very heavy standard resistance 

 rere available having so large a cross-section that the current density due to a 



