382 Intelligence and Miscellaneous Articles. 



astonishing discrepancies ; and I think it useful to draw the attention 

 of physicists to this subject, which has been but little studied. 



A simple method of measuring the horizontal intensity consists 

 in passing a constant current through a circuit containing a weight 

 voltameter, and a tangent-galvanometer. By determining the 

 absolute strength of the current on the one hand by the results of 

 electrolysis, and on the other by the deflection of a compass- 

 needle, and equating these two values, we can get the value of T at 

 the spot where the galvanometer was placed. This method was 

 of sufficient exactitude for the work of comparison in which I was 

 engaged. 



A Poggendorff's battery may be used ; this is a very constant 

 source when the chromic liquid is strongly acid, and the external 

 resistance is great. As an electrolyte I took a 10 per cent, 

 solution of pure copper sulphate ; the copper electrodes at a 

 distance of about 30 mm. had 12 square centimetres immersed ; 

 from this resulted a favourable density of current, and therefore a 

 beautiful deposit of metal which was continuous and perfectly 

 adherent. The loss of the soluble electrode was always equal to 

 within 5 mgr. to the gain of the negative electrode. The intensity 

 of the current, which was about -J- of an ampere, was determined to 

 within go^o of an ampere ; it was assumed that 1190 mgr. was 

 deposited per ampere-hour. Two good tangent-galvanometers 

 were used simultaneously ; their constants are as follows : — 





Leng 



th 



Mean 



Number 





Galva- 

 nometer. 



x 





radius. 

 E. 



of 

 windings. 



R. 



of needle 



of wire 



27TTO 





mm. 



mm. 



cm. 







A .... 



15 



1258 



16-68 



12 



0-221 



B .... 



20 



1114 



16-12 



11 



0-233 



The needles are suspended to a cocoon- thread ; the long pointers 

 of aluminum enable us to read -^ of a degree. The relative 

 dimensions of the needles and of the frames are in these two 

 instruments in such a ratio, that we may dispense with the use of 

 the term of correction, which I have considered proportional to the 

 tangent of the deflection I. 



The manipulation was very simple; the element having been 

 shortcircuited for a few minutes, the current was passed for an 

 hour through the voltameter and the galvanometer. Two double 

 readings were made after five and twenty-five minutes ; the current 

 being reversed in the galvanometer after thirty-five and fifty-five 

 minutes. The mean of these eight readings gives the value of the 

 mean deflection of the needle in the course of the operation. It 

 remained to weigh the electrodes, and to take the mean p of the 

 loss and gain of the plates in milligrammes. The formula 



i._^__-^-T=tana 



10 1190 ~" 2im 



leads to the value of T in C.Gr.S. units; and the same operations 

 repeated in various places enable us to discover considerable 



