as a Standard of Electromotive Force. 3 



when arranged as shown ; but of course it depends greatly on 

 the position of the tubes, and also somewhat on the tempera- 

 ture. Hence it is not to be regarded as giving necessarily a 

 very constant current, but rather as a cell which can be used 

 for a long time and yet keep its electromotive force nearly un- 

 changed. 



I have also made a set of small cells on the same principle, 

 with ordinary quilled tubing for the tubes, and with test-tubes 

 for the containing vessels, making the connexions by twisting 

 the thin copper wire of one cell round the projecting tube 

 (with the zinc bent down springily over it) of the next. A 

 large number of such cells may be quickly made and arranged 

 in ordinary test-tube stands ; and they are convenient for 

 many purposes, such as capacity- or insulation-testing, where 

 high electromotive force is required*. The whole rack of 

 cells was once accidentally upset ; but though a little liquid 

 escaped from the open ends of the zinc-tubes, the copper-liquid 

 remained steady at the bottom of its tube without visible dis- 

 turbance. 



A Cell for a Standard of Electromotive Force. 



Fig. 2 shows a bottle about 3 inches high, which I have 

 made to act as a standard of electromotive force. It differs in 

 no essential respect from fig. 1, except that the mouth of the 

 tube containing the copper-solution never dips below the sur- 

 face of the liquid, but always projects \ inch above it. The 

 other or open tube does not project at all above the cork ; and 

 its lower end is drawn out and coiled round so as still further 

 to retard the passage of the copper-liquid to the zinc. The 

 zinc, which should be pure, is supported at the right height 

 by a pin thrust through it. The closed tube is proportionally 

 longer than in fig. 1 ; it is nearly filled with pure sulphate-of- 

 copper solution, a few crystals being placed at the bottom ; 

 and it is tied to the other tube, as before, with silk thread 

 (which appears not to rot). The copper wire is gutta-percha- 

 covered with its ends bared. The bottle is filled nearly to its 

 neck with very dilute sulphate of zinc ; and the cork is then 

 inserted air-tight. 



No mixing of the liquids is now possible ; but conduction 

 still takes place over the damp surface of the glass tube, espe- 

 cially if, before use, the whole bottle be slightly inclined so as 

 to wet the edges of the tube. The slight film of zinc-salt thus 

 formed, being hygroscopic and being in a saturated atmo- 



* I suppose that by using platinum instead of copper wire, and strong 

 nitric acid or else sulphuric acid and bichromate of potash instead of the 

 copper salt, one could nearly double the electromotive force, though with 

 some loss of constancy. 



B2 



