1872.] Voltaic Standard of Electromotive Force. 



445 



a thoroughly saturated solution of zinc sulphate, the positive element con- 

 sisting of pure zinc resting on the paste. The best method of forming this ele- 

 ment is to dissolve pure zinc sulphate to saturation in boiling distilled water. 

 When cool, the solution is poured off from the crystals and mixed to a thick 

 paste with pure mercurous sulphate, which is again boiled to drive off any air ; 

 this paste is then poured on to the surface of the mercury previously 

 heated in a suitable glass cell ; a piece of pure zinc is then suspended in 

 the paste, and the vessel may be advantageously sealed up with melted 

 paraffine-wax. Contact with the mercury may be made by means of a 

 platinum wire passing down a glass tube, cemented to the inside of the cell, 

 and dipping below the surface of the mercury, or more conveniently by a 

 small external glass tube blown on to the cell, and opening into it close to 

 the bottom. The mercurous sulphate (Hg 2 S0 4 ) can be obtained com- 

 mercially * ; but it may be prepared by dissolving pure mercury in excess 

 in hot sulphuric acid at a temperature below the boiling-point : the salt, 

 which is a nearly insoluble white powder, should be well washed in distilled 

 water, and care should be taken to obtain it free from the mercuric sulphate 

 (persulphate), the presence of which may be known by the salt turning 

 yellowish on the addition of water. 



The electromotive force of the elements thus formed is remarkably 

 uniform and constant, provided the elements be not connected up and 

 allowed to become weak by working. A long series of comparisons was 

 made between various elements, some of which had been made many 

 months, and it was found that the greatest variation among them all did 

 not differ from the mean value more than one thousandth part of the 

 whole electromotive force ; such a large difference was, however, very un- 

 usual, and might have been due to slight differences of temperature. 



Several experiments were made to determine the variation of the electro- 

 motive force produced by temperature, from the mean of which it appears 

 that the electromotive force decreases with increased temperature in the 

 ratio of about *06 per cent, for each degree Centigrade ; for example, au 

 element gave relative values of '9993 at 0° Cent, and "9412 at 100° C, 

 between which limits the decrease appeared nearly proportional to the in- 

 crements of temperature. These results, however, might be verified with 

 advantage. 



The element is not intended for the production of currents, for it falls 

 immediately in force if allowed to work on short circuit. It is intended to 

 be used only as a standard of electromotive force with which other ele- 

 ments can be compared by the use of the electrometer or condenser, or 

 other means not requiring the use of a prolonged current. The author 

 finds that the most delicate method of making these measurements is by 

 means of his potentiometerf. 



* The author has obtained it from Messrs. Hopkin arid Williams, 5 New Cavendish 

 Street. 



t See 'A Treatise on Electrical Measurement,' by Latimer Clark. London, 1868, p. 10C. 



