288 Messrs. J. H. Gray and J. B. Henderson. 



PART IT. 



By JAMES H. GRAY and JAMES B. HENDERSON. 

 Tests on Change of Electrical Resistance. 



Several methods were tried with more or less success at the begin- 

 ning of the work of Part I. Great difficulty was at first experienced 

 with thermo-electric currents and the self-induction of some of the 

 coils used. Ultimately, the zero method about to be described was 

 perfected, and was used in the work of Part II, giving great satis- 

 faction. 



Before describing this method, we wish to refer particularly to the 

 two definitions of specific resistance at present employed. The unit 

 most generally understood in English treatises is the resistance in 

 ohms of a cube of the metal of unit section and unit length. From 

 this definition we have, for the resistance R, of a length Z, of a 

 homogeneous conductor of uniform section w, 



where a v is the specific resistance so defined. a v may be called the 

 " volume specific resistance," in contradistinction to the " weight 

 specific resistance " <r w , which is defined from the following. We 

 have 



T? l l * l i 



JK, = ff v = a v n- - r= ff w l, 



u} UUp 10 



where <r v p = tr w , w being the weight of the length I of the wire. 

 From this, a w is seen to be the resistance of a length of the wire 

 numerically equal to p and section unity, or of a length unity and 

 section equal to !//>. Since the section is uniform, Z/w, or the length 

 per unit of weight, is constant. Let it be represented by X. Then 

 we have 



B = ff w \l. 



The advantages of using this latter equation over the one involving 

 the " volume specific resistance " are very many, either when it is 

 required to know the whole resistance of a wire, having given <r w , A, 

 and Z, or when it is required to know <r w , having given B, X and Z. 



It is quite usual in commercial circles to speak of a wire of, say, 

 number 14 gauge, weighing 127 grains to the foot, that is, about 

 27 grammes to the metre. The only measurement to be taken then 

 is the length, if the specific resistance be given in weight units, and 

 the measurement of the length can be made with the greatest 

 accuracy. Even if the weight per unit of length be not given, it can 

 .also be determined most accurately without any difficulty. 



If, however, the specific resistance be given in volume units, the 



