﻿[ 157 ] 

 XXIII. Intelligence and Miscellaneous Articles. 



ON SOME HYDRO- AND THERMOELECTRIC FORCES, REDUCED TO 

 SIEMENS^ UNIT OF RESISTANCE AND WEBER'S UNIT OF CUR- 

 RENT. BY F. KOHLRAUSCH. 



T HAVE made the present research in conjunction with M. A. Am- 

 ■*- mann. It refers to the electromotive forces of Grove's and of 

 Daniell's elements, to the combination copper and zinc in dilute sul- 

 phuric acid, and the thermo-elements German silver and copper, cop- 

 per and iron, and German silver and iron. Where it is not other- 

 wise specially mentioned, the determinations were made by Poggen- 

 dorff's compensation method. The electromotive forces are throughout 

 expressed according to Ohm's law, e=wi, where the resistance to is 

 expressed in Siemens's units, and the intensity i in the magnetic unit 

 introduced by Weber. The electromotive forces thus measured I 

 shall express by Siemens-Weber. 



To measure the electromotive forces of the hydroelectric circuit, a 

 tangent-compass of twenty-four turns was used, the mean diameter 

 of which was 258*4 millims. The coils formed a circuit with a rect- 

 angular section 27 millims. in breadth and 9*4 millims. in height. 

 The magnetic needle was a rectangular magnet 20 millims. in length 

 with pointers affixed. 



I will here give a formula for such a tangent-compass for the 

 correction of the first degree, which can be frequently applied in 

 galvanometrical measurements. 



If r is the mean diameter of n circular coils which together form a 

 circuit with a rectangular section of the breadth 2a and the height 

 2b, if, finally, 21 is the distance of the poles of the needle from each 

 other (meaning by pole the centre of the free unipolar magnetism), 

 then the intensity of the current may be expressed in magnetic mea- 

 sure by the formula 



. rT/ lx l« 2 lb 2 3 / 2 V /. . 15 P . 2 \ 



a= ( 1 + — - — — - — -—J tan (pi 1 + rSir* ) 



2me\ 2r 2 3 r 2 4 r 2 / r \ 4 r 2 r J' 



T signifies here the horizontal intensity of the earth's magnetism, 

 and <j> the angle of deflection which the current i produces. It is 

 presupposed that a, b, and / are small compared with r. The inten- 

 sity of the earth's magnetism at the place of observation was compared 

 with that in this magnetic observatory, and thence determined from 

 the secular formula for Gottingen. T was then found =1-902. 

 The distance of the poles was assumed to be 19 millims. Hence for 

 this tangent-compass 



2=1-631 tan 0(1 +0-020 sin 2 0). 



A Siemens's scale was used as rheostat, it having been compared 

 with our standards. 



The electromotive forces were found by the method of compensa- 

 tion to be as follows : — 



1. Grove's element (that is, platinum, concentrated nitric acid, 

 sulphuric acid of 1*06 sp. gr., freshly amalgamated zinc) 



= 19*98 Siemens- Weber. 



