Standard Resistance-coils with Mercury Standards. 347 



It was found that the resistance of the contact pieces re- 

 mained very nearly constant during the time occupied in 

 making a series of comparisons. The variations in the values 

 found before and after rarely amounted to more than 1 bridge- 

 wire division, or about '00005 ohm; and the mean of the 

 two values obtained is pretty certainly correct to less than 

 this. 



At the same time different fillings of the same contact-tubes 

 led to very different values for their resistances. This was 

 no doubt due to small irregularities in the contact between 

 the platinum and the mercury. Thus I found the following 

 values in terms of a bridge-wire division : — 



Date &c. 



Resistance of contacts. 



March 25 



93-4 b.w.d. 

 69-4 „ 



68-2 „ 



114-5 „ 

 109-4 „ 



,, Contacts refilled 



o/» /Copper rods being left in the \ 

 \ mercury overnight J 



„ „ Contacts refilled, one renewed 



„ 28 Copper rods being left in since 26th. . . 



The differences between the 69*4 and the 68*2, and, again, 

 between the 114*5 and 109*4, are due, I believe, to the copper 

 of the contact-rods being dissolved by the mercury in the tubes. 



On March 28th, the contact-tubes were cleaned and refilled, 

 and the resistance was found to be 57*2 b.w.d. Throughout 

 the experiments until April 9, when one of them was broken, 

 their values did not differ greatly from the above. 



On April 1 1th, new contact-tubes were prepared and used 

 throughout the rest of the experiments; their value remained 

 fairly constant at about 58 b.w.d. 



Thus the quantity actually determined by the observations 

 was the difference between the resistance of the mercury tube 

 + the contact pieces and the standard coil. In order to 

 get the resistance of the mercury tube in terms of the standard 

 coil, the resistance contact-pieces found as above described 

 had to be subtracted. 



Another way of eliminating the resistance of the contacts 

 from the result is to compare first the mercury tube and then 

 the standard with some other coil, using the same contact- 

 pieces to connect the mercury tube and the standard to the 

 bridge. This can be done by removing the contact-tubes 

 from the mercury tube, and dipping them into two mercury 

 cups, into which the electrodes of the standard also dip. Since, 



