GALVANIC INSTRUMENTS. 



333 



will maintain an equal action for many hours. To the 

 rod, or cylinder of zinc, and also to the copper cylinder, 

 are attached brass cups with binding screws, forming the 

 poles of the battery. The zinc is negative, and the copper 

 positive. The exterior copper vessel is usually about six 

 inches high, and four wide ; the porous tubes are filled with 

 dilute sulphuric acid (about one part of acid to one of water) ; 

 and the copper cylinders are tilled with a strong solution of 

 sulphate of copper, also acidulated by sulphuric acid ; so 

 that the acid in the generating cell is separated from the 

 solution of sulphate of copper, but the porosity of the tubes 

 allows of their becoming so far imbued with the acid liquid 

 as to admit of the passage of electricity. Price, $1.75. 



Porous Cells for Galvanic Batteries. The most important 

 modification of the galvanic battery, is that of the introduc- 

 tion of porous cells, forming what is termed a constant 

 battery. In all the other arrangements the electrical power 

 is liable to fluctuation, and for many investigations are incon- 

 venient and even useless ; by the introduction of these cells 

 those inconveniences are to a great extent obviated, and al- 

 though it is more complicated, its con- Figs. 393, 

 stant and regular action, when it is pro- ^JH^ 331, 

 perly constructed, amply repays the 

 additional trouble and expense. 



They consist of cells, or cups, of un- 

 glazed porcelain (Figs. 393, 394, 395), 

 of such sizes as may suit the battery for 



393. 



Fig. 396. 



which they are intended ; and though they separate the 

 acid in the outer cell from the inner part, the porosity of 

 the porcelain allows sufficient to penetrate through to pro- 

 duce a considerable galvanic action. 



Price, small size, lOcts. each. 

 " large " 12icts. " 

 " for DanielFs battery, 



25 cts. each. 



DanielVs Six-cell Sustain- 

 ing Battery. (Fig 396.) 

 The annexed cut represents 

 a set of six of the above 

 batteries. The copper cy- 

 linder is connected in this 



