THE SMITHSONIAN INSTITUTION. 327 



Let lis now examine the process for obtaining the readings of the 

 rheometer with this unit. 



§ 8. Determination of the force of a current by its chemical effects. — 

 To reduce the magnetic action cf the current in the rheometer to the 

 chemical effect, the current has only to be passed simultaneously 

 through a decomposing apparatus and the rheometer ; a voltameter 

 which gives the two gases together a detonating mixture, is the 

 best adapted for this purpose. 



A current which, for instance, passed through a Mohr's torsion 

 galvanometer, and a decomposing apparatus, produced 40 cubic cen- 

 timetres of detonating gas per minute, while the corresponding torsion 

 of the galvanometer amounted to 490°. 



Since the torsion is in this instrument proportional to the force of 

 the current, w^e should have, for forming one cubic centimetre of the 

 gas, a current corresponding to a torsion of Yo° =^ 12°. 2 . . . ; or 

 each degree of torsion should be equivalent to ^Vo = 0.0816 cubic 

 centimetres of detonating gas. To reduce the number of degrees read 

 on this galvanometer to Jacobi's unit, the former need only be mul- 

 tiplied by 0.0816. Hence a torsion of v° is equivalent to the force 

 0.0816 V. 



The process is exactly the same for reducing the data of the tangent 

 compass to the chemical effect. In such an instrument, for instance, 

 a deflection of 22° was observed, while 30.8 cubic centimetres of gas 

 were developed. The temperature being 15° Centigrade and the 

 height of the barometer 740 millimetres, the quantity of this gas 

 reduced to 0° Centigrade and a pressure of 760 millimetres is 28.18 

 cubic centimetres. 



Since in this instrument the forces of currents are proportional to 

 the tangent of the angle of deflection, the tangent of 22° or 0.404 

 corresponds to the quantity of gas, 28.18 ; and the tangent 1 corre- 

 sponds to the quantity J.^^^^f = 69.7 ; thus the tangent of any angle 

 of deflection read on this instrument has to be multiplied by 69.7 to 

 find out how many cubic centimetres of detonating gas the current 

 would have produced per minute, if it had passed with the same force 

 through a decomposing apparatus ; hence the force 69.7 fang, v cor- 

 responds to the angle of deflection v, according to our chemical unit. 



It is easy to reduce the indication of a compass of sines to this unit in 

 a similar manner. 



The factor by which the indications of a rheometer are to be mul- 

 tiplied, to obtain the force of current expressed in chemical measure, 

 must of course be determined with great accuracy, for which a single 

 experiment is not sufficient ; a series of experiments must be made 

 with currents of different forces, computing the factor from each, and 

 from the values thus obtained the mean is to be taken. The different 

 current forces are most easily obtained by operating, first with a 

 battery producing a strong decomposition of water, and then weak- 

 ening the current by removing single elements at a time. 



Such a series, instituted by Mohr with his torsion galvanometer, 

 gave the following results : 



