DILUTE SOLUTIONS AT THE FREEZING POINT. 



331 



being in electrical connection with each other, and with one of two brass rings which 

 are fitted in vulcanite beds on to the axis of the drum. A pair of wire brushes are 

 brought into contact with the brass rings, and another pair touch the strips as they 

 revolve. The leads from a single dry cell are connected with the first pair, and the 

 second pair are led to the places on a box of resistance coils at which the lottery 

 wires are usually attached in the Wheatstone's bridge arrangement The drum was 

 at first made to revolve by means of an electric motor ; a hand-wheel was after- 

 wards found more convenient. Thus the current from the battery is made to 

 alternate rapidly before it reaches the box of coils. 



Fig. 5. 



The other end of the drum is arranged in an exactly similar manner, except that the 

 brass strips are rather narrower, and the intervening areas of vulcanite rather broader. 

 One pair of brushes is connected with the galvanometer terminals of the Wheatstone's 

 bridge, and the other pair with the terminals of the galvanometer itself. Thus the 

 galvanometer connections are alternated synchronously with those of the battery, the 

 galvanometer being taken out of circuit a little before the battery, and thrown into 

 circuit a little later. By this means the current, which alternates while it is passing 

 through the electrolytic cell, is again made direct before it reaches the galvanometer, 

 the deflection of which can therefore be used as an indication of the direction in 

 which the resistances must be changed in order to get a balance. 



The galvanometer used was made on the D' Arson val principle, and was constructed 

 by Messrs. Nalder Bros. This type has important advantages over the Thomson 

 form. It is quite sensitive enough for the present purpose; its freedom from magnetic 

 disturbances is useful ; above all, the large moment of inertia and slow period of 

 swing of the suspended portions prevent small residual periodic disturbances from 

 annoying the observer. This enables very low resistances to be measured. 



When thus arranged the method is very satisfactory. All periodic disturbances, 



2 TJ 2 



