446 G. 0. Squier — Electro- Chemical 



effects of a variation of the magnetic field could be observed 

 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 

 volts. The connections with the compensating circuit, 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 resist- 

 ance 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 deflections 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 deflections with 

 the test-coil were taken for simple " make " or " break " and 

 not for reversed field, thus eliminating the residual magnetism 

 of the pole-pieces. 



In the formula applicable, viz : — , — — „ = — nearly, in 



H' 7tna d' J 



which d and d' represent the deflections due to the Inductor 



and test-coil respectively ; H and IT the earth's field and the 



field to be measured ; n and n' the number of turns and a 



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



mm* = 207l6 sq cm 



TTn'a'" = 6-788 sq cm 



d = 4-697 



Distance between pole-pieces 3-5 cm . 



W = 1299-48 <#'H, and as d' varied from ■£$ to 16, the range 

 of field employed was from 65 to 20,800 H. 



A curve was constructed so that from accurate ammeter 

 readings in the field circuit, the strength in absolute measure 

 could be read off at once. 



Experimental Results. 



Preliminary. — The first experiments were made with very 

 dilute nitric acid and iron electrodes, one a circular disc of 

 5 mm radius, and the other a small wire l cm long and l mm in 

 diameter turned to a sharp point at one end. The point was 

 placed opposite the center of the disc, at a distance of l cm 

 from it, and the whole placed so that the cylindrical electrode 

 coincided with the direction of the lines of force. When the 

 minute point and the center of the disc were exposed to the 



