210 



Profs. W. E. Ayrton and John Perry. [Mar. 21, 



It is to be observed that although, there was a motion of the elec- 

 trometer needle when the quadrants were insulated (shown by the 

 great difference between every two successive readings), still the 

 observations are sufficiently consistent, and when taken two by two 

 they show that polarization has the effect of decreasing the readings. 

 Taking the means of every two successive readings we have in the 

 zinc sulphate, 11" 75, 8*3, 8"4, and again, after the zinc is cleaned, 

 10*75, 10*8, 9*0. In the acid this effect is even more marked, tke 

 deflections diminishing rapidly from 10 - to 3 '9, and again from 9 "15 

 to 6'0. Thus we see that polarization in the ordinary sense, so far 

 from producing a difference of potentials between zinc and copper 

 joined by a liquid, in reality tends to reduce the difference of potentials 

 that already existed between them. 



The electrometer now began to show the existence of two faults, 

 imperfect contact between the electrodes and the quadrants, aud a 

 slight discharge from the needle to the quadrants. The instrument 

 was, therefore, taken to pieces, and the first fault rectified by the 

 addition of contact springs (in addition to the little weights previously 

 employed) between the electrodes and quadrants, and the second by 

 the needle and quadrants being cleaned. 



In observations made during the last four months on the constancy 

 of electromotive force of cells, we found that Latimer Clark's cell 

 when tested by an electrometer (kept in a perfectly constant state) 

 altered very little when its poles were kept insulated. The electro- 

 motive force of this cell is known to be 1*457 volts, and we have used 

 it as a standard to determine the changes produced in readings by 

 changes in the positions of the quadrants. 



When we compared the electromotive force of contact of zinc and 

 copper with the constant cell, it was found in every case to be 0*75 

 volt at the temperature of 17° C. The deflections in the last two 

 tables might be reduced to volts if we knew the contact electromotive 

 force of zinc and copper at 10° C. 



Table Till.— 13th April, 1876. Plates 10 mm. apart. 

 Latimer Clark's Cell. 



Zero. Reading. Deflection. 



955 -0 892 -0 63 -0 



954 -5 1018 -5 64 -0 



954 -5 891 -8 62 7 



953 -1 1017 -1 64 -0 



Mean 63 "4 



Assumed to be 1 "457 volts. 

 Direct reading is 355 

 355 



Therefore ratio is — - — - or 5 '6. 



63 '4 



