546 



DANIELL, SMEE, GRENNET, AND LECLANCHE'S ELEMENT. 



copper, filled with a saturated solution of cupric sulphate (fig. 382). A roll of eopper, perforated 

 with a few holes, is placed in the copper solution, and in order that the latter be kept saturated, 

 and to supply the place of the copper used up by the battery when in 

 ^ action, there is a small shelf on the copper roll, on which are placed 

 crystals of cupric sulphate. A porous earthenware vessel containing 

 zinc in contact with dilute sulphuric acid (1 : 7) is placed within the 

 copper cylinder. When the circuit is completed, the zinc is acted on, 

 zinc sulphate being formed, and hydrogen liberated. The hydrogen in 

 statu nascendi passes through the porous cell, reduces the cupric sulphate 

 to metallic copper, which is precipitated on the copper cylinder, so that 

 the latter is always kept bright and clean. The liberated sulphuric 

 acid replaces that in contact with the zinc. Owing to the absence of 

 polarisation, the Daniell is one of the most constant batteries, and is 

 generally taken as the standard of comparison.] 

 Fig. 382. [The copper is the + pole, zinc the - .] 



Daniell s cell. [Smee's cell contains only one fluid, viz., dilute sulphuric acid 



<1 : 7), in which the two metals, zinc and platinum, or zinc and platinised silver, are placed. 

 The platinum is the + pole, and zinc the - .] 

 [Grennet's or the Bichromate cell consists of one plate of zinc and two plates of compressed 

 rarbon in a fluid, containing bichromate of potash, sulphuric acid, and water. The fluid con- 

 sists of 1 part of potassium bichromate 

 dissolved in 8 parts of water, to which 

 one part of sulphuric acid is added. 

 Measure by weight. The cell consists 

 of a wide-mouthed glass bottle (fig. 



Fig- 383. Fig. 384. 



Fig. 383. Grennet's cell. A, glass vessel; K, K, carbon; Z, zinc; D, E, binding screws for 

 the wires; B, rod to raise the zinc from the fluid ; C, screw to fix B. Fig. 384. 

 Leclanche's cell. A, outer vessel ; T, porous cylinder, containing K, carbon ; B, binding 

 screw ; Z, zinc ; C, binding screw of negative pole. 

 383); the carbons remain in the fluid, while the ziuc can be raised or depressed. When 

 not in action, the zinc, which is attached to a rod (B), is lifted out of the fluid. It is not a 

 very constant battery. When in action, the zinc is acted on by the sulphuric acid, hydrogen 

 being liberated, which reduces the bichromate of potash. 



The carbon is the + pole, and the zinc the - .] 

 [Leclanche's cell (fig. 384) consists of an outer glass vessel containing zinc in a solution of 



