ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 14& 



this fact, because making the electrolysis in this manner one has 

 certainly to take a greater figure for the equivalent, and therefore the 

 ratio between the unit and the measured amount of current remains the 

 same. The following figures are given by taking one ohm=l-063 S.U., 

 and by assuming that a current of one ampere strength deposits in an 

 hour 4-0259 grammes ; the last figure exceeds that given by the Board 

 of Trade only by six parts in 100,000. I found by some thirty experi- 

 ments the E.M.F. of the H cells, set up with clean materials in the 

 above- described manner, as 1-4332 volt at 15°, and am sure that, when 

 using the same arrangement of the silver voltameter, this value will be 

 right by five parts in 10,000 if the equivalent is certain to this extent. 

 If I express the value given by Lord Rayleigh for the cells of the original 

 Clark's form in this unit, it is 1-4346 volt at 15°. Lord Rayleigh finds 

 the E.M.F. of this H cell a few ten-thousandths of a volt greater than 

 that of the old form. Therefore it would, perhaps, be 1-4350 volt at 15°i 

 Recently Mr. Glazebrook has made a new determination, and finds the 

 E.M.F. of the original Clark cell, in the above fixed units, to be 1-4342 volt 

 at 15°. He has also compared H cells set up by me, and now brought 

 to England, and finds their E.M.F. smaller by Twoot^'s of a volt than 

 that of the original form. Therefore the E.M.F. of the H cell is 1-4338 

 volt at 15°. This last value and that found by me are in good agree- 

 ment. It is to be noted that the anodes and cathodes in my voltameters 

 are much smaller than those in the English ones. 



It may be mentioned here that the mean E.M.F. of four H cells set 

 up in the same manner as before, but containing, in accord with Professor 

 Carhart's directions, a solution of sulphate of zinc, saturated at 0°, was 

 found to be 1-442 volt at 16°, using the same units as above. 



It only remains to give some directions on the best form of Clark 

 cells._ I suppose it will be good to distinguish such cells which are to 

 remain as standards in the laboratories and are used by their maker, and 

 such as are to be used for practical purposes. These do not need to have 

 the same degree of accuracy, but they must be able to be carried about. 

 In the Board of Trade memorandum the original Clark cell is adopted as 

 the standard ; but I think Lord Rayleigh's H form gives more accu- 

 racy and is easier to set up. In the old form not all the parts of the 

 zinc rod are in saturated solution, and therefore the value of the E.M.F. 

 will be a little uncertain. Another disadvantage is that parts of the zinc 

 rod may fall down in the mercury, and will so produce a considerable 

 variation of the E.M.F. On the other hand, the electrodes of the H form 

 are always in concentrated solution, and there is no possibility of parts 

 of the negative electrode coming over to the positive one. I have set np 

 about sixty H cells, and have found no difficulty, when using carefully 

 cleaned materials, to keep the difference of the E.M.F. of the single cells 

 under a ten-thousandth of a volt. 



To construct cells for practical purposes which will stand carriage, 

 the most simple way is to separate the two electrodes by a porous wall. 

 I can show here such a cell of a form constructed by Dr. Feussner, and 

 issued hitherto by the German Reichsanstalt. The positive electrode is 

 an amalgamated platinum plate with the surrounding paste in a porous 

 vessel of clay. The zinc rod forming the positive electrode is on the 

 upper part protected by a glass tube ; the lower part is blown rectan- 

 gularly and covered with crystals of sulphate of zinc. The whole glass 

 vessel is filled with a concentrated solution of this salt. The E.M.F. of 



