﻿Sir W. Thomson on a Constant Form 0/ Darnell's Battery. 541 



An extemporized cell on this plan was exhibited to the Royal 

 Society ; and its resistance (measured as an illustration of Mance's 

 method, described in the first of his two previous communications) 

 was found to be -29 of an Ohm (that is to say, 290,000,000 centi- 

 metres per second). The copper and zinc plates of this cell, being 

 circular, were about 30 centimetres in diameter, and the distance 

 between them was about 7'o centimetres. 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 re- 

 sistance 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 resist- 

 ance may be diminished by diminishing the distance rather than 

 increasing the surface, it is impossible to deduce from the 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 with- 

 out 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 wheelwork will be avoided by the introduction of a 

 comparatively inexpensive electromagnetic engine. For electric bells, 

 and all telegraphic testing and signaling on shore, the new form of 

 battery will probably be found easier of management, less expen- 

 sive, 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 be 

 found the most convenient form ; but the new form is certainly 

 better for all ordinary shore uses. 



The accompanying drawing represents a design suitable for the 

 electric light, or other purposes, for which an interior resistance not 

 exceeding ^ of an Ohm is desired. The zinc is in the form of a 



grating, to prevent the lodgment of bubbles of hydrogen gas, which I 

 find constantly, but very slowly, gathering upon the zincs of the 

 cells I have tried, although the solutions used have no free acid, 



