On the Comparison of Resistances. 399 



I. A simple plan of securing the facility of interchange of 

 contacts is to arrange four mercury- cups in a row, as 1, 2, 3, 4 

 in fig. 1, and connect them, as shown in the figure, with the 

 binding-screws A B and C D of the bridge by stout copper rods. 

 The mercury-cups should have thick copper bottoms, or, if the 

 arrangement is to be permanent, may be formed conveniently 

 of short bars of copper surmounted by pieces of india-rubber 

 tube, so that connecting-rods can then be permanently fixed 

 to the copper bars. The electrodes of the coils X and Y are 

 inserted at 1 and 3, and 2 and 4 respectively, or vice versa, and 

 their connexions can accordingly be interchanged with very 

 little motion. The electrodes can be kept in contact with the 

 bottoms of the mercury-cups by elastic bands. 



This plan of adapting the bridge differs but little from 

 that described by Prof. Thompson, but is, perhaps, somewhat 

 simpler. 



II. The second method which we have been in the habit of 

 using requires a more elaborate apparatus, but has the advan- 

 tage that the contacts of the electrodes of the coils themselves 

 are not altered during an experiment, the interchange being 

 effected by means of a key. The coils under examination 

 can, indeed, be soldered to projecting parts of the key if 

 necessary, and the requisite contacts are secured by springs 

 pressing down metal tongues into mercury-cups. The advan- 

 tage of this arrangement is very considerable when experi- 

 ments are being made by observers who are unfamiliar with 

 the eccentricities of contacts. 



The arrangement of the key is shown in plan in fig. 2, and 

 in section in fig. 3. Eight mercury-cups 1, 2, 3,4, 5, 6, 7, 8 

 are formed by boring holes in an ebonite plate, about half an 

 inch thick, and screwing copper plates on the back (shown by 

 dotted lines in the figure). These copper plates, which are 

 carefully insulated from each other, connect the eight mercury- 

 cups in pairs, and are brought out in tongues A, B, C, D, to 

 which rods, forming connexions with the binding-screws of 

 the bridge, can be soldered, or temporary contacts can be 

 made by means of the mercury-cups formed by drilling the 

 ebonite, as shown in the figure. 



To another plate of ebonite, fixed opposite to these cups on the 

 same board, four pairs of thick tongues of copper — I, II, III, 

 IV, V, VI, VII, VIII — are attached, the ends of the tongues 

 forming springs about 6 inches long. At the end of each 

 spring a copper plunger is arranged to dip down to the bottom 

 of the corresponding mercury-cup, into which it is pressed by 

 a spring, S, as shown in fig. 3. The springs work against the 



