400 Mr. S.W.J. Smith on a 



are o£ equal size, and the capillary passes from the middle. of: 

 one to the middle of the other. The apparatus contains 

 approximately enough mercury to completely fill one limb, 

 and about half as much sulphuric acid solution. By suitably 

 adjusting the distribution of the mercury and the solution in 

 the two limbs, the apparatus can be arranged for use with 

 the capillary tube either horizontal or tilted upwards at a 

 considerable angle. The maximum angle of tilt available is 

 increased by increasing the lengths of the limbs in com- 

 parison with the length of the capillary. It is easy to 

 construct an electrometer which can be used with the capillary 

 at almost any inclination between horizontal and vertical. 



The distribution of the mercury can be altered most easily 

 by means of a cross-piece provided with a tap, as represented 

 in the figure by dotted lines. When the tap is open there 

 is free communication between the mercury in the two limbs, 

 and the relative amount in each can be altered by tilting 

 the apparatus. When the tap is closed the two quantities of 

 mercury are insulated from each other. The addition of this 

 arrangement tends to complicate the construction of the 

 instrument, and, although it is very convenient in practice, 

 it is not indispensable. Any desired changes in the distribu- 

 tion of the mercury and the solution can usually be effected 

 without much trouble, by means of the capillary and the 

 upper cross-tube. 



To prevent the platinum wires forming the electrodes being 

 wetted by the acid solution, if the apparatus should acci- 

 dentally, or during transit, be laid on its side or turned up- 

 side down, the lower ends of the tubes may be drawn out, 

 as shown at P/ and P 2 X in the second figure, and slightly 

 constricted above the ends of the platinum terminals. This 

 precaution may be dispensed with if the electrodes are formed 

 by welding pieces of platinum foil, which are afterwards 

 amalgamated, on to the ends of the wires F 1 and P 2 . The 

 apparatus can then be turned upside down and shaken, so 

 that the amalgamated foil becomes wetted by the acid, with- 

 out any further ill-effect being produced than an occasional 

 slight wandering of the zero of the instrument for a short 1 

 time after it is righted again. 



The usual spring-key has several disadvantages. Thus, if 

 it is made of brass, the contacts frequently become unsatis- 

 factory through surface tarnishing, and if, to avoid this, the 

 bearing surfaces are made of platinum the key sometimes 

 shows pronounced thermoelectric effects. Further — and this' 

 is a point of some importance in a portable instrument — the 

 key cannot conveniently be fastened on to the same stand as 



