256 



Sir William Thomson on a 



[Jan. 19, 



to replace the liquid drawn off by the siphon from near the bottom. A 

 cover may be advantageously added above, to prevent evaporation. When 

 the cell is much used, so that zinc enough is dissolved, the liquid added 

 above may be pure water; or if large internal resistance is not objected to, 

 the liquid added may be pure water, whether the cell has been much used 

 or not ; but after any interval, during which the battery has not been 

 much in use, the liquid added ought to be quarter saturated, or even 

 stronger solution of sulphate of zinc, when it is desired to keep down the 

 internal resistance. It is probable that one or more specific-gravity beads 

 kept constantly floating between top and bottom of the heterogeneous 

 fluid will be found a useful adjunct, to guide in judging whether to fill up 

 with pure water or with sulphate-of-zinc solution. They may be kept in a 

 place convenient for observation by caging them in a vertical glass tube 

 perforated sufficiently to secure equal density in the horizontal layers of 

 liquid, to be tested by the floaters. 



An extemporized cell on this plan was exhibited to the Royal Society, 

 and its resistance (measured as an illustration of Mance's method, de- 

 scribed in the first of his two previous communications) was found to be 

 •29 of an Ohm (that is to say, 290,000,000 centimetres per second). The 

 copper and zinc plates of this cell, being circular, were about 30 cen- 

 timetres in diameter, and the distance between them was about 7*5 centi- 

 metres. A Grove's cell, of such dimensions that forty in series would give 

 an excellent electric light, was also measured for resistance, and found to 

 be *19 of an Ohm. Its intensity was found to be 1*8 times that of the 

 new cell, which is the usual ratio of Grove's to Daniell's ; hence seventy- 

 two of the new cells would have the intensity of forty of Grove's. But the 

 resistance of the seventy-two in series would be 209 Ohms, as against 76 

 Ohms of the forty Grove's ; hence, to get as powerful an electric light, 

 threefold surface, or else diminished resistance by diminished distance of 

 the plates, would be required. How much the resistance may be dimi- 

 nished by diminishing the distance rather than increasing the surface, it is 

 impossible to deduce from experiments hitherto made. 



Two or three cells, such as the one shown to the Royal Society, will be 

 amply sufficient to drive a large ordinary turret-clock without a weight ; 

 and the expense of maintaining them will be very small in comparison 

 with that of winding the clock. The prime cost of the heavy wheel- work 

 will be avoided by the introduction of a comparatively inexpensive electro- 

 magnetic engine. For electric bells, and all telegraphic testing and signal- 

 ing on shore, the new form of battery will probably be found easier of 

 management, less expensive, and more trustworthy than any of the forms 

 of battery hitherto used. For use at sea, it is probable that the sawdust 

 Daniell's, first introduced on board the 'Agamemnon' in 1858, and ever 

 since that time very much used both at sea and on shore, will still probably 

 be found the most convenient form : but the new form is certainly better 

 for all ordinary shore uses. 



