Table 40 (continued) 87 



PROBABLE VALUES OF THE GENERAL PHYSICAL CONSTANTS 



The most accurate direct determination of the mechanical equivalent of 

 heat / is the work of Laby and Hercus, 1 which appeared since the H.P. was 

 compiled. They use a continuous flow calorimeter and make 23 determinations, 

 grouped about six different temperatures, the temperature change in the 

 calorimeter being always about 5°C. Their result is 7 = 4.1841 ±0.0001 abs. 

 joules at i6.67°C 



A more precise method of reduction is first to adopt a curve for the tempera- 

 ture variation of the specific heat of water. Such a curve is given immediately 

 by eq. (1). If it is desired that the specific heat at I5°C be unity, eq. (1) is to 

 be divided by 4.18327. Doctor Birge finally adopts 



one 15° calorie (J 1B ) =4.1852 ±0.0006 abs. joules 

 one 15° " (J' 15 ) =4.1835 ±0.0007 int. joules 

 and by eq. ( 1 ) j^ _ 4 lgl3 ± Q 00Q6 abs joules 



J' 20 = 4.1796 ± 0.0007 int. joules 



The faraday (F). — The faraday is defined as the quantity of electricity 

 carried in electrolysis by one gram equivalent of any element. It is believed to 

 be a general constant of nature. According to modern ideas, each univalent ion 

 carries a charge numerically equal to the electronic charge e. The Avogadro 

 number N gives the number of atoms (or molecules) in one gram equivalent. 

 Hence one may define the faraday more precisely as the product N • e. The 

 fact that F can be most accurately evaluated from electrolysis, and N is then 

 obtained from F and e, does not affect the validity of the definition. 



One electrochemical equivalent is the mass associated with unit electric 

 charge. Like the faraday, its true value, independent of experimental condi- 

 tions, depends only on the adopted unit of charge. On the other hand we can 

 measure only the amount of a substance deposited or released in an electrolytic 

 cell, per unit current per second. This is affected by experimental conditions, 

 and may or may not equal the electrochemical equivalent. The faraday is 

 then, by definition, the ratio of the gram equivalent of a substance to its 

 electrochemical equivalent. Almost universally the distinction between mass 

 deposited per unit charge, and electrochemical equivalent is ignored. Con- 

 siderable confusion results regarding the best value of certain electrochemical 

 equivalents, and the resulting best value of the faraday. 



Nevertheless, it is convenient to assume, for the moment, that the silver 

 deposited per unit charge in a silver voltameter, under the conditions defining 

 the international ampere, is the electrochemical equivalent of silver {E^g)- 

 With this assumption, the value of faraday follows from constants already 

 adopted. The gram equivalent of silver, or of any univalent substance, is 

 numerically equal to its atomic weight in grams (Ag). The amount of silver 

 deposited in electrolysis by one international coulomb is, by definition, 

 0.00111800 gram. Hence 



1 Philos. Trans., A 227, 63, 1927. 

 Smithsonian Tables 



