294 On the Measurement of Small Resistances. 



Table giving Results of Experiments. 



No. of cells. 



Bridge-read- 

 ing for P. 



Bridge-read- 

 ing for P'. 



Pv in ohms. 



Resistance 



added to arms 



of bridge. 



e in volts. 



3 Leclanches ... 

 2 do., multiple 



mm. 

 5012 



501-2 

 4961 

 5446 



705*5 

 5008 

 507-5 



mm. 

 5019 



5102 

 472-9 

 526-3 

 644-0 

 4675 

 489-0 



2-02 



2-02 



•04 



2-02 



2-02 



•02 



•04 















22 



5-2 











75-10" 7 



60-10- 7 

 75-10- 7 

 65-10 -7 

 63-10" 7 

 60-10" 7 

 63-10 -7 



1 Leclanclie 



1 Leclanclie 



1 Leclanclie 



IDaniell 



1 Daniell 





Thus the mean value of e as given by the experiments with 

 the Leclanche cells is 7 7' 6 x 10 -7 volt, while that as given 

 by the Daniells is 61 "5 x 10 -7 volt. The wire of the bridge 

 and the contact piece are platinum-iridium ; the other wires 

 are copper. 



The closeness in the values of e would appear to show that 

 the assumption of some action at this junction will account for 

 most of the phenomena ; while the constancy of the effect 

 shows that, if due to a difference of temperature between the 

 bridge-wire and the contact piece, that difference cannot vary 

 much. 



The hand of the experimenter was separated from the metal 

 of the contact piece by a cap of wood. The effect of main- 

 taining contact for some time after finding the equilibrium- 

 position was tried without result. If the junctions A and C 

 of the platinum- wire and the battery circuit be heated, the 

 effect is decreased ; if they be cooled, it is increased. Thus 

 the effect, if thermoelectric, is due to heating at D. 



In making measurements of resistance, this effect can 

 of course be eliminated in various ways ; it appears, 

 however, to be worth while to call attention to its existence. 

 One method frequently adopted to correct for this thermo- 

 electric effect is given by Prof. Chrystal and Mr. Saunders 

 (Report on the Standard Units of Resistance, B.A. Report 

 1876, p. 13). They made contact w T ith the sliding piece 

 before making the battery-contact, and waited until the thermo- 

 electric current had attained a steady value and the galvano- 

 meter-needle come to rest in a position slightly disturbed from 

 its original one. If on making the battery-contact no effect was 

 produced, the resistance is given by the ordinary equation 



