THE STANDARD OF POTENTIAL. 20, 



In other words, the electromotive force of our standard has fallen off 

 several tenths of I per cent from the value required by the Reichsanstalt 

 formula for a Latimer-Clark cell at this temperature. Several days later, in 

 reply to some inquiries concerning the variability of Clark cells of this type, 

 Professor Peirce made a second very instructive investigation of this and 

 other similar cells. He wrote as follows : 



... I first tested against a certain cell used as a makeweight the four Muirhead 

 standard cells, 10915I, 13361I, 10915^ I336ir, received a few months ago from the 

 Cromptons to serve as standard units, with a very elaborate potentiometer made and 

 tested by them. The readings were 1.4223, 1.4236, 1.4219, 1-4243- the cells being at 

 22.5 C. Your cell, which had been at 20.6 for some time, gave a reading of 1.4226 

 and a cadmium cell gave 1.0191. You will notice the comparatively large difference 

 between the English cells, which are from a famous maker and have been tested by a 

 well-known firm of electrical engineers. If 10915, as is probable, was made before 

 13361, the fall of E. M. F. with age seems to be indicated. 



Of the Clark-Carhart cells A and B are seven or eight years old, I think; 264 and 

 265 are, I believe, more than six years old; 702, 706, 735, 737, and 740 are new (the 

 oldest bears the date February 18, 1902, and carries a guarantee with proper treatment 

 for three years, but I can not be sure that the laboratory has never fallen below allow- 

 able temperatures). The potentiometer readings were, at temperature in cells of 23.5 : 



A 1.4257 265 1.3939 735 1-4335 



B 1.4202 702 1.4337 737 1-4335 



264 1.4260 706 1.43 14 740 1-4325 



The effect of age or wear and tear due to temperature changes and not to polariza- 

 tion in use seems clear. 



Our cadmium cells are all almost exactly alike, and if for the sake of argument we 

 assume that the E. M. F. of any one of them at room temperatures was 1.0185 true 

 volts, and apply to them the slight temperature correction called for by the formula, 

 the best two Muirhead cells would have E. M. F.s of about 1.4241 and 1.4248 at 21.5 , 

 where the formula calls for 1.4248 true volts. The best three Carhart-Clark cells would 

 have at 21.5 E. M. F.s of about 1.434 instead of 1.436, and your cell would have at 

 21.5 , the E. M. F. 1.4206, which was nearly what I got the other day. 



Professor Peirce's exhaustive work establishes our potential standard be- 

 yond the possibility of doubt. 



The formula used for the temperature coefficient 7r ( o=7r 15 o o.ooi2( 15) 

 is substantially the same as that given by the Reichsanstalt. 



The comparison of the i-volt cell with the standard Clark element was 

 made on the standardized potentiometer already described, in the usual way. 



After the reading, the i-volt cell was returned to its original position to 

 prove that the potential of the Leclanche cell had remained constant. 



Professor Peirce's value for the Clark cell, corrected for its temperature 

 coefficient, time-lag being eliminated as nearly as possible, established the 

 potential of the i-volt cell as follows: 



