74 Mr. W. C. Baker on the Hall Effect in 



u areas/' A was placed between the poles of a magnet and 

 the current set up in the secondary circuit, due to the action 

 of the magnet on the primary current, was balanced against 

 the current in the shunt-circuit, by altering the resistance R 

 until the galvanometer spot remained at rest on closing the 

 key K. 



The advantage of this arrangement is that as the key K is 

 closed only for a small fraction of a second, it permits the 

 galvanometer to indicate at once whether the shunt current 

 is greater or less than that in the secondary, but the primary 

 current is stopped again before the complicated thermo- 

 magnetic effects of von Ettingshausen and Nernst * have had 

 time to rise to significant proportions. By thus obtaining 

 the value of the compensating current in the shunt-circuit 

 for both directions of the primary current and for both 

 directions of the magnetic flux in each case, all effects but 

 the one here sought are eliminated in the meanf. 



Gold was chosen to work with chiefly because it could be 

 obtained of a standard purity, in thin sheets, much more 

 readily than other metals. The foil was procured from a 

 local dentist, and its thickness was determined from its mass, 

 area, and the density of beaten gold (19*367, Watts' ' Dic- 

 tionary of Chemistry'). These sheets were found to vary in 

 thickness, even for different parts of the same piece ; so in 

 the final determination of the absolute value of the effect, the 

 portion extending from one secondary electrode to the other 

 was cut out, and its thickness determined after it had been 

 thoroughly cleaned, first in alcohol, to remove the shellac, 

 and then in strong nitric acid. 



Much trouble was experienced in obtaining a successful 

 connexion to the gold. Attempts to solder it to brass strips 

 failed completely, as the thin foil dissolved in the hot solder, 

 and, by its surface-tension, the alloy drew away from the 

 gold, leaving absolutely no contact. Systems of clamps, such 

 as Hall J used in his original investigation, and the various 

 modifications that were tried, all proved to be too variable 

 to be used with the small resistances employed. 



The method finally adopted is shown in fig. 2. Two 

 comb-shaped bits of copper (D D) were screwed to a piece 

 of ebonite about 12 cms. square and 5 mms. thick, the copper 

 " teeth " being previously well tinned. The gold-foil A 

 (10 cms. square) could just lie flat on the ebonite between 



* Wied. Ann. xxix. p. 344. 



•f" For a full discussion of how each separate effect is eliminated by 

 taking the mean of these four readings, see Lebret'e paper, loc. cit. 

 X Phil. Mag. vol. x. p. 301 (1880). 



