CHAP. IIL] ELECTRICITY AND MAGNETISM. 141 



nitric acid in order to appear at the platinum pole, de- 

 composes the nitric acid and is itself oxidized, producing 

 water and the red fumes of nitric peroxide gas. This 

 gas does not, however, produce polarisation, for as it is 

 very soluble in nitric acid it does not form a film upon 

 the face of the platinum plate, nor does it, like hydrogen, 

 set up an opposing electromotive -force with the zinc. 

 The Grove cells may be made of a flat shape, the zinc 

 being bent up so as to embrace the flat porous cell on 

 both sides. This reduces the internal resistance, which 

 is already small on account of the good conducting 

 powers of nitric acid. Hence the Grove's cell will 

 furnish for three or four hours continuously a powerful 

 current. The E.M.F. of one cell is about 1-9 volts. A 

 single cell will readily raise to a bright red heat two or 

 three inches of thin platinum wire, or drive a small 

 electro -magnetic engine. For producing larger effects 

 a number of cells must be joined up " in series," the 

 platinum of one cell being clamped to the zinc of the 

 next to it. Fifty such cells, each holding about a quart 

 of liquid, amply suffice to produce an electric light, as 

 will be explained in Lesson XXXII. 



172. Bunsen's Battery. The battery which bears 

 Bunsen's name is a modification of that of Grove, and 

 was indeed originally suggested by him. In the Bunsen 

 cell the expensive 1 platinum foil is replaced by a rod or 

 slab of hard gas carbon. The difficulty of cutting this 

 into thin slabs causes a cylindrical form of battery, .with 

 a rod of carbon, as shown in Fig. 74, to be preferred to 

 the flat form. The difference of potentials for a zinc- 

 carbon combination is a little higher than for a zinc- 

 platinum one, which is an advantage ; but the Bunsen 

 cell is more trouble to keep in order, and there is some 

 difficulty in making a good contact between the rough 



1 Platinum costs about 30 shillings an ounce nearly half as much as gold ; 

 while a hundredweight of the gas carbon may be had for a mere trifle, often 

 for nothing more than the cost of carrying it from the gasworks. 



