1878.] 



Contact Theory of Voltaic Action. 



201 



vessels of liquid, contact is made between the liquids by wetting a 

 piece of cotton wick in one of the liquids and allowing its ends to dip 

 below the liquid surfaces. 



The brass rods N 1 , insulated where they pass through the wooden 

 frame by glass tubes and bone washers, are gauges which may be 

 lowered or raised until their ends are at fixed distances below the level 

 of the plates A and B. By the reflections of these ends at the surfaces 

 of the liquids or plates, we can judge of the position of the level sur- 

 faces G and D. The wooden pegs and 0', and a vertical slot in the 

 stand at WW", enable a reversal of G and D to be made with accuracy 

 and rapidity. 



The parts of the frame WWW" in view of the brass plates are 

 covered with tin plates or tin foil, connected with the outside of the 

 electrometer. A tin plate door, of which the hinges are seen at MM, 

 shuts up a metal box enclosing the whole apparatus, including the 

 electrometer, short-circuit key, and connecting wires. This box is 

 permanently in connexion with the outside of the electrometer. The 

 plug Z of the short-circuit key is kept connected with this cover 

 through a hole in which it may be withdrawn from its bed to insulate 

 the plates. 



We shall now describe a complete operation, to obtain the electro- 

 motive force of contact between a metal and a liquid, for example. 

 Suppose the permanent adjustments to have been made, and that the 

 plates A and B are quite bright. The plate D is cleaned and laid on 

 the levelling screws B ; the porcelain dish containing the liquid is laid 

 on the levelling screws Q. By turning 8 and the levelling screws 

 (0 moving in its slot), and by the use of a gauge gh of which the h 

 part represents the distance apart determined upon for A and C and 

 B and D, we get the surfaces into their proper relative positions. The 

 brass rod N' is now lowered until its end and its reflection at the sur- 

 face of D seem to meet, and it is then made fast. Before proceeding 

 further, the insulation of the glass rods, the permanence of the zero 

 of the electrometer, and the permanence of position of the needle 

 when the quadrants are insulated, are ascertained. The strip E which 

 is quite bright is now bent to dip into the liquid, and the cover M is 

 lowered. Insulate by means of the key, and take the zero. Now, lift 

 the cover M, lower the table and reverse G and D ; raise the table 

 until the end of N and its reflection in the surface of the liquid nearly 

 meet ; lower the cover and take a reading. Now, short-circuit the 

 quadrants, insulate, and proceed to take a new reading. Alternate 

 readings are on opposite sides of the zero. 



"We found that when slight changes occur in the dryness of the air 

 inside the electrometer, there are changes in the zero which are probably 

 due to slight traces of moisture remaining on the suspension fibres of 

 the needle. It is also to be remarked that when the quadrants of the 



