36 



Dr. G. Gore. 



[Dec. 18, 



with wire to prevent leakage. As it often requires considerable 

 pressure to force the mercury through the tube, especially when there 

 exists both a mechanical obstacle and a stoppage of the electric 

 current, a strong pressure screw, with its clamp capable of grasping at 

 least an inch in length of the chamber, is absolutely necessary. 



I have tried several plans of mechanically compensating the electro- 

 capillary effect, the most convenient of which is to surmount the 

 pressure- chamber by a vertical glass tube about 30 or 40 centims. 

 high, and 1 millim. bore, with its lower end opening into the chamber,, 

 and its upper end also open, but capable of being closed by meaus of 

 a glass cock, having an open cup at its upper extremity. This 

 arrangement admits of great pressure being applied to the chamber,, 

 by means of the screw and clamp, and also of crudely measuring in 

 millimetres height of mercury the compensating pressure necessary to 

 depress the mercury to the neutral point whilst the current is passing.. 

 This upper tube also, like the capillary one, has the disadvantage of 

 offering unequal resistance to the passage of the mercury by unequal 

 adhesion, so that a small movement of the screw of the pressure- 

 chamber is not often at once attended by a simultaneous movement of 

 the mercury at the top and bottom of the column. 



The voltaic current was very conveniently manipulated by the aid 

 of a small moveable apparatus, so constructed that by turning its 

 handle the first quarter of a revolution, the two wires of the electro- 

 scope were connected metallically together, and the current insulated 

 from them ; by turning it a second quarter, the current was sent in 

 one direction through the electroscope; by turning it a third, the 

 wires of the electroscope were disconnected and perfectly insulated 

 (the base of the instrument being of ebonite) ; and by turning it the 

 fourth quarter, the current was sent through the electroscope in a 

 reverse direction. This apparatus may be easily made by prolonging 

 the axis of a common electric reverser, fixing two projecting metal 

 studs on opposite sides of the prolonged part (but not opposite to each 

 other), and attaching two vertical springs to two binding-screws fixed 

 on the base, so that when the studs pressed against the upper ends of 

 these springs, the circuit was closed between the ends of the wires 

 attached to the two electrodes. 



In the engraving, fig. 1 represents the reverser fixed vertically 

 to a moveable wooden stand. The dotted lines show the directions of 

 four connecting wires at the back of the ebonite base of the instru- 

 ment. The wires from the voltaic cell are attached to two binding- 

 screws, which are connected with the two metal standards that 

 support the moveable axis. The terminal wires of the electroscope 

 are attached by means of binding screws to two vertical brass springs, 

 so fixed as to press against the two metal studs projecting at right 

 angles from the moveable axis ; the terminals are also further con- 



