1885.] arrangement for maintaining a constant high potential. 227 



making the work which the motor has to do proportional to a 

 high power of the speed. This is most easily obtained by setting 

 the motor to drive a fan. If then we arrange a motor with a con- 

 stant head of water in connection with a fan, we can by adjusting 

 the water supply, and the dimensions of the fan, obtain a tolerably 

 wide range over which the speed can be made constant at any 

 desired point. Let a motor and fan therefore be fitted up : and 

 thus governed let it drive the arrangement shewn in the diagram- 

 matic drawing. 



A copper disc is placed between the poles of an electro- 

 magnet, this disc is made to rotate with a tolerably high velocity 

 by means of the governed motor. As long as no current is flowing 

 round the magnet the energy spent in rotating the disc is very 

 small. 



Iu the circuit there is mica or glass condenser of considerable 

 capacity ; its actual dimensions and construction depend on the 

 potential at which the apparatus is desired to be maintained, and 

 on its capacity. If the capacity of the apparatus is itself large 

 then no condenser will be necessary. There is a pully which 

 is driven by a much smaller one on the same shaft as the copper 

 disc, and is connected with a replenishing guage of a type to be 

 determined by the special conditions of the experimental apparatus. 

 For instance, the value of the desired potential will determine what 

 precautions must be taken in insulating and this will determine 

 the size of the replenisher. The electrical output required to 

 compensate the fall of potential which it is desired to guard 

 against will determine the further construction of the replenisher. 



A fixed disc is in connection with that pole of the apparatus 

 which it is desired to maintain at a constant high potential, 

 another disc is suspended by a long and fine spiral spring which 

 in its turn is hung to a wire the other end of which goes to a 

 Weber suspension mounted on the top of the guard tube, and not 

 shewn in the diagram. The usual method of levelling the 

 suspended plate is adopted. Since there is no need for any parti- 

 cular accuracy of levelling this may be easily accomplished. The 

 suspended plate carries a wire whose ends are bent down vertically 

 and tipped with aluminium. Below the ends of these wires are 

 placed two mercury cups on adjustable stands. The object of 

 using aluminium joints is to avoid the capillary effects so noticeable 

 in the case of platinum or any amalgamated contact breakers. 

 Mercury is generally stated to wet aluminium without amalgam- 

 ating with it, but from some experiments which I made on the point 

 I have concluded that it can only be with certain specimens of 

 aluminium that this takes place ; with ordinary foil and wire 

 I invariably found that the mercury is always on the point of 

 wetting the aluminium but never quite succeeds. The advantage 



