TRANSACTIONS OF SECTION A. 511 



experiments ; the test applied being a microscopic deflection of a sort of electro- 

 meter needle suspended inside a coated <^lass beaker. A charged body in the neigh- 

 bourhood is suddenly discharged or reversed, or else rapidly moved. The coat of 

 silver on the beaker is found to protect, only so long as it is opaque ; it no longer 

 does so when the deposit is thin enough to be transparent. 



I 



8. On a new Form of Current-Wei cjher. 

 By Professor James Blytu, M.A., F.B.S.E. 



In the construction of balances for the measurement of electric currents a 

 greater or less difficulty has always been experienced in leading the current into 

 the movable parts of the instrument without seriously interfering with its sensi- 

 bility. Several methods have been adopted to overcome this ditHculty. In some 

 balances the current is led in by mercury cups ; in others, flexible leads, made of 

 thin wire spirals or thin metal strips, are employed ; while, in the recent balances by 

 Sir William Thomson, the dilliculty has been overcome by means of his well- 

 known ligature suspension. 



Some time ago it occurred to me that still another form of balance might be 

 employed for this purpose, and the present paper is a short description of one which 

 I have made. 



The balance referred to is of the ordinary Roberval type, with the pivot con- 

 nections all replaced by tightly-stretched torsion wires. 



It is constructed as follows : — 



On a flat base-board are placed two vertical uprights of wood or other insu- 

 lating material, about G inches apart. Between these are stretched two parallel 

 wii-es in the same vertical plane about 3 inches apart. To the middle points of 

 these wires are soldered the two horizontal metal bars of the Roberval. 



These are about 9 inches long. Both horizontal bars terminate at each end 

 either in forks or rings, placed in a horizontal plane, and wires are tightly stretched 

 between the prongs of the forks, or across a diameter of the rings. To the middle 

 points of these last wires are attached, also by soldering, the vertical bars of the 

 balance, thus completing what takes the place of the ordinary jointed parallelo- 

 gram of the Roberval. 



The vertical bars have metal terminals, insulated from each other, and carry 

 the circular discs, on the rims of which the movable coils of wire are wound. 

 The bars pass at right angles through the centres of the discs, and are fixed to them 

 at their middle points. The fixed coils, which are of exactly the same diameter 

 as the movable ones, are supported from the base-board, and are placed so that one 

 is about half an inch above one movable coil, wliile the other is as much below the 

 other movable coil. From this it will be seen that when the balance is in equih- 

 brium the fixed and movable coils are all horizontal, with a space of about half an 

 inch between each pair. 



The stretched wires may be either of steel or phosphor-bronze, and before 

 being finally soldered are placed under considerable tension. 



The current flows through the instrument thus : — entering, say, by the upper 

 wire connecting the fixed supports, it passes to the upper horizontal bar. There it 

 splits into two, one half (supposing the resistances equal) passing to each end of the 

 bar, and, by means of the coiTesponding fork-wires, passing through the movable 

 coils. From the movable coils each half returns along the lower horizontal bar 

 and together pass out by the lower wire connecting the two main supports. From 

 this the whole current passes first through the one fixed coil and then throu"-h 

 the other, and in such a direction as to produce a repulsion between each pair of 

 coils. 



In the constructing care is taken that the suspended coils are both made of 

 equal weiglit, and that when the balance is in equilibrium no torsion is in any of 

 the ^s-ires. Small scale-pans are attached to each vertical bar, and a bob for 

 raising or lowering the centre of gravity of the whole is placed on a rod springing 

 «t jight anirles from the middle of one of the horizontal bars. 



The current strength is estimated by the weight needed to restore the balance 



