28 ENERGY CHANGES INVOLVED IN DILUTION OF AMALGAMS. 



The most serious sources of accidental error partial short circuiting of 

 points differing- widely in potential, and thermoelectric currents were con- 

 stantly borne in mind. All switches and keys were mounted on glass, and 

 all wires not carried in air lines were encased in glass tubes throughout their 

 entire length. The apparatus was set up in a dry basement room which 

 responded slowly to outside temperature changes. The absence of thermo- 

 electric effects had to be inferred rather than proved. Brass, copper, and 

 manganin, which have very small thermoelectric forces against each other, 45 

 formed most of the circuit, while platinum and German silver were sparingly 

 used, and opposite junctions with other metals were always close together. 

 Finally, the disposition of the apparatus seemed to offer no opportunity for 

 large permanent temperature differences at various points. 



Boxes A and D were of manganin ; A was at least eight years old and D 

 almost as well aged. Hence the relative values of the resistances can be 

 assumed as constant over long periods. 



THE STANDARD OF POTENTIAL. 



All was now read) 7 to measure the potential of concentration cells with an 

 actual error not greater than five millionths of the i-volt cell. Evidently the 

 latter's value in absolute units must be known as accurately as possible. The 

 standard at first relied upon was a large Clark cell of the usual form, bearing 

 the stamp of the Reichsanstalt. As it had been in the laboratory for six 

 years, it seemed wise to verify the statement of its potential as found in its 

 certificate. Immediately after the close of the measurements on zinc, Pro- 

 fessor B. O. Peirce, of the Department of Physics in Harvard University, 

 examined it with great care, a service for which we are deeply indebted. 

 His conclusions are quoted from his letter as follows : 



A hasty comparison made on Saturday of your normal Clark element with two 

 standard cadmium cells, presumed to have the E. M. F. of 1.0186 volts at 21 , gave as 

 the E. M. F. of the former 1.420-}- at 21.5 . A more careful comparison made to-day 

 gave 1.4206 with each cell at 21. 5 , and since the element has been kept at practically a 

 constant temperature for some time, the effect of time-lag may be assumed to have 

 disappeared. 



I tested also five Carhart-Clark cells, and found that these differed considerably 

 among themselves. As the E. M. F. of such a cell is likely to fall off, as times goes on, 

 I chose that which had the highest E. M. F., and using Carhart's certificate of 1.4400 

 0.00056 (t 15) as the E. M. F., got the same value as before (1.4206 international 

 volts at 21.5 ) for the E. M. F. of your element. 



Landolt und Bornstein (Meyerhoffer) Tabellen, p. 776 (1905). 



