476 Lieut. Gr. 0. Squier on the Electrochemical 



Carnegie, Phipps, and Co., of Pittsburg, and were practically 

 pure. 



In order to insure a uniform density of surface, the elec- 

 trodes were turned from the same piece and polished equally 

 with fine emery-cloth. The magnet could be made or 

 reversed at the galvanometer, and its strength varied at will 

 by a non-inductive resistance. The electrochemical effects 

 due to the magnetic field could thus be studied with facility 

 by the fluctuations of the galvanometer-needle. The original 

 difference of potential, which always existed between the 

 electrodes, was compensated by a fraction of a Daniell cell, 

 so the effects of a variation of the magnetic field could be ob- 

 served when no original current was passing between the 

 electrodes. 



The standard cells were made with care, and under 

 uniform treatment possessed at 20° C. an electromotive force 

 of 1*105 volt. The connexions with the compensating cir- 

 cuit, which contained a finely-divided bridge, were so 

 arranged that from its readings the difference of potential 

 between the distant electrodes became known at once without 

 involving the resistance of the cell or of the galvanometer. 



Since quantitative measurements of the effects observed 

 were desired, a preliminary step was to calibrate the electro- 

 magnet for a given distance apart of the pole-pieces. The 

 method employed was the well-known one of comparing the 

 galvanometer deflexions produced by a test-coil in the field 

 with those of an earth inductor in series in the circuit. 

 Since the effect of the sudden addition of a certain strength 

 of field was wanted instead of its absolute value, the de- 

 flexions with the test-coil were taken for simple "make" 

 or " break " and not for reversed field, thus eliminating th< 

 residual magnetism of the pole-pieces. 



In the formula applicable, viz., 



H irna 2 d , 



H'W^ = d' nearl y' 



in which d and d' represent the deflexions due to the 

 inductor and test-coil respectively, H and H' the earth's 

 field and the field to be measured, n and ri the number of 

 turns, and a and a' the radii of the coils, the particular values 

 were :— - 



irna 2 — 20716 square centim. 

 7ni'a ,2 = 6'788 square centim. 

 ^ = 4-697. 



Distance between pole-pieces 3*5 centim. 





