Modified Resistance-Balance. 509 



same effect as if X and S were bodily interchanged ; and 

 without lifting either of those coils as in Fleming's instrument. 

 Balance having been once more obtained, and a new reading 

 x r of the scale having been taken, the length x — xf, multiplied 

 by the resistance per centim. of the divided wire, is equal to 

 the actual difference between X and S. 



The switch-board part is made of such dimensions that it 

 can be attached on to any of the ordinary pattern of metre- 

 bridges. The ordinary metre-balance is, however, limited in 

 its application, so far as Foster's method of comparison is 

 concerned, to the comparison of resistances which do not 

 differ by a greater amount from one another than the actual 

 resistance of the wire of the rheocord part. To facilitate 

 the comparison of coils of high resistance whose resistances 

 differ by several ohms, I have adopted the following arrange- 

 ment of the rheocord part. Two wires are selected, each 

 two metres long, one having a resistance of about a quarter 

 of an ohm, the other from eight to ten ohms. They are 

 stretched parallel upon a board 213 centim. in length, 

 provided with a two-metre scale, which should be divided 

 into millimetres. Either of these wires can be used at will, 

 connexion being made to either at one end of the bridge by 

 a stout copper link dipping into mercury-cups drilled in the 

 wood and lined with copper. Stout copper rods of about 1 

 square centim. section lead from the ends of the stretched 

 wires to the screws, by which the switch-board part is con- 

 nected with the rheocord part. The galvanometer-contact 

 is made by means of a sliding key shown separately in fig. 3, 

 which is provided with a vernier, and which by a simple 

 arrangement can be set so as to make the contact with either 

 the thick or the thin wire, as may be desired. In the instru- 

 ment exhibited to the Society the stout wire is of german- 

 silver 2*1 millim. in diameter, and of 0'2385 ohms resistance. 

 The thin wire is of silver-platinum alloy; its diameter is 033 

 millim., and its resistance 8' 21 ohms. The form of slider 

 adopted is a much more well-conditioned arrangement than 

 the usual heavy medal sliding key standing on three legs, 

 and is more convenient in practice. The greater length of 

 rheocord, though it renders the instrument less portable, 

 increases the range of the instrument without affecting its 

 sensitiveness. 



Phil. Mag. S. 5. No. 109. Suppl. Vol. 17. 2 M 



