for the Measurement of Small Currents: 93 



tends very little to diffuse upward. The kathode was a rod 

 of amalgamated zinc, loosely jacketed with filter-paper or 

 linen. The zinc rod should be supported so that it only dips 

 a few centimetres into the top of the liquid, thus diminishing 

 the liability of any of the liberated iodine coming in contact 

 with it. The jacketing was found necessary to prevent par- 

 ticles of electro-deposited zinc becoming detached and falling 

 into the solution of iodine at the bottom of the cell. 



When used with currents not exceeding -^o °f au ampere 

 and an anode of 9 square centimetres, it was found that no 

 iodine diffused up to the level of the kathode in experiments 

 of over an hour's duration, the cell being kept completely at 

 rest. With feebler currents, lasting for a protracted time, it 

 was found advisable to use a U-tube with a good plug of 

 asbestos in the bend, filled with the zinc-iodide solution and 

 having a platinum plate and zinc rod in the two sides of the 

 tube ; but naturally with this form the resistance was much 

 greater than with the beaker form. 



With large currents u electric convection " leads to diffusion 

 of the iodine through the whole liquid, and therefore the 

 author does not consider it well adapted for their measure- 

 ment, except for approximate, rapid determinations. 



In the Report to the Board of Trade of the Electrical 

 Standards Committee, the importance is pointed out, in using 

 a silver voltameter, of having a considerable ztf^-resistance in ' 

 the circuit to mitigate the effect of change of resistance in 

 the voltameter itself : the same precautions apply to the iodine 

 voltameter and a slight readjustment of the resistance at first 

 employed is usually necessary to keep the current, as 

 indicated by a galvanometer, constant. This is owing to a 

 small change in the adverse electromotive force, but if a 

 battery of fairly high E.M.F. (ex. gr. 3 Grove cells) be em- 

 ployed, controlled by a moderately large resistance, the current 

 may be kept very constant. 



After the iodine voltameter has been in circuit for a time 

 which will vary with the degree of accuracy required and the 

 actual strength of the current, the latter is stopped and the 

 zinc kathode is immediately removed from the liquid. The 

 solution is then stirred up, and the amount of free iodine 

 determined by direct titration with standard sodium thiosul- 

 phate solution, after the addition of starch. Taking the 

 electrochemical equivalent of iodine as '001314: grm. per 

 coulomb, a convenient strength of sodium thiosulphate solu- 

 tion is one of which each cubic centimetre reduces five times 

 this amount (1 cub. centim. = '00657 grm. I) : this corre- 



