﻿Liquid Films between Iridio-Platinum Electrodes. 319 



On A and B are fitted two caps of ebonite clamped on by 



screws a, 1>. Mounted on each ebonite is a sheath of steel. 

 On the left cap is a plate of iridio-platinuin /;, and on the 

 right is a disk of iridio-platinuin q, set on a spindle turning 

 on an axis in the plane of the paper and perpendicular to B. 

 This disk projects, as shown, so that when B advances to A, 

 the surfaces q and p are brought into contact. Since q is a 

 diametral section of a sphere, its edge touches the plate p at 

 a point. Discharge is made to occur between p and q. By 

 causing p to rotate on a horizontal axis along A and q to 

 rotate on a horizontal axis perpendicular to B, fresh surfaces 

 of p and q can be presented to one another continually, since 

 q touches p not at the centre. 



The P.D. on surfaces/?, q is maintained by connecting wires 

 to terminals d e. The full circuit is shown in fig. 2. Here a 

 400- volt lighting circuit is joined through key K 4 to a water 

 resistance. Between the fixed zinc electrodes Z,, Z 3 is the 

 full P.D., while between the moveable Z 2 and the fixed Z t is 

 the P.D., say V, used in discharging the apparatus. The 

 leads to the electrodes Z 2 and Z 3 are insulated from the 

 liquids by being enclosed in glass tubing. 



If switches K 2 and K 3 are put over to the right, the above 

 voltage V will act through the voltmeter V or on the dis- 

 charge surfaces p, q according as the keys K 5 and K 6 are put 

 over to the left or the right. 



It is easy to vary the P.D. used by moving Z 2 . 



Another circuit is on the left and is brought into action 

 hy putting switches K 2 and K 3 over to the left, whilst K 5 

 and K 6 are kept to the right. The single cell C has resistance 

 ?'!, say 100 ohms, and ?\ 2 , say 4000 ohms, in circuit. The 

 telephone T is in the circuit and sounds when p and q touch. 

 This circuit is used to determine the position of contact of 

 p and q, the P.D. between them beinu- very small, in this 

 case ^(j volt. The actual discharge for high potential experi- 

 ments, say 100 volts, from the right circuit, takes place when 

 p and q are separated by a small gap ; but the discharge- 

 distance for the small voltage ^ l volt of the left circuit is so 

 minute that it can be ignored. The above apparatus can be 

 used for discharge through gases, liquids, or solids. 



The modus operandi for liquids is as follows : — The caps 

 <on spindles A B are removed from the spindles, the iridio- 

 platinum surfaces ground with finest emery to remove the 

 old surface, polished with rouge or leather, and cleaned. The 

 caps are placed on A and B and p, q made to approach. One 

 drop of the liquid to be tested is placed on q, and as p 

 -approaches q the liquid settles by capillarity between p and q. 



