VOLTAIC COMBINATIONS. 



metallic elements of tlie combination are thin rectangular slips 

 cut from sheet metal. The form, however, is in no manner 

 essential to the production of the electric current. So long as 

 the magnitude of the surfaces exposed to contact with the solution 

 is the same, the current will have the same force. The pieces of 

 metal may therefore have the form here supposed of thin rec- 

 tangular plates, or they may be formed, as is often found con- 

 venient, into hollow cylinders, that of the copper being so much 

 less in diameter than that of the zinc, that it is capable of being 

 placed within it without mutual contact. 



The simple arrangement first adopted by Yolta consisted of 

 two equal discs of metal, one of zinc, and the other of copper or 

 silver, with a disc of cloth or bibulous card, soaked in an acid or 

 saline solution, between them. These were usually laid, with 

 their surfaces horizontal, one upon the other. 



The late Dr. Wollaston proposed an arrangement, in which the 

 copper plate was bent into two parallel plates, a space between 

 them being left for the insertion of the zinc plate, the contact of 

 the plates being prevented by the interposition of bits of cork or 

 other non-conductor. The system thus combined was immersed 

 in dilute acid, contained in a porcelain vessel. 



Dr. Hare of Philadelphia contrived a voltaic arrangement, con- 

 sisting of two metallic plates, one of zinc and the other of copper, 

 of equal length, rolled together in the form of a spiral, a space of 

 a quarter of an inch being left between them. They are main- 

 tained parallel without touching, by means of a wooden cross at 

 top and bottom, in which notches are provided at proper distances, 

 into which the plates are inserted, the two crosses having a 

 common axis. This combination is let into a glass or porcelain 

 cylindrical vessel of corresponding magnitude, containing the 

 exciting liquid. 



This arrangement has the great advantage of providing a very 

 considerable electro-motive surface with a very small volume. 



The exciting liquid recommended for these batteries when great 

 power is desired, is a solution in water of 2 1 per cent, of sulphuric, 

 o.nd 2 per cent, of nitric acid. A less intense but more durable 

 action may be obtained by a solution of common salt, or of 3 to 5 

 per cent, of sulphuric acid only. 



31. It is not essential that the water in which the metals are 

 immersed be acidulated, as we have supposed, by sulphuric acid. 

 Any acid which will promote the oxydation of the zinc without 

 affecting the copper will answer. Nor is it indeed necessary that 

 any acid whatever be used. A saline solution is often found more 

 convenient. Thus common salt dissolved in the water will 

 produce the desired effect. 



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