VOLTAIC BATTERIES. 



55 



in action for a lengthened period of time. It consists of a cell 

 of copper, which of itself forms the negative metal, containing 

 a tube of porous earthenware of much smaller diameter. Within 

 this porous tube is placed a rod of amalgamated zinc, to which, 

 as also to the copper cell, is attached a binding screw. A cell 

 of this description is excited in the following way : the porous 

 tube containing the zinc is filled with dilute sulphuric of the 

 ordinary strength, and into the copper cell is poured a saturated 

 solution of sulphate of copper, made by pouring boiling water 

 upon a superabundance of the crystals of the salt of copper. 

 A perforated metal shelf is fitted to the top of the cell, for the 

 support of a supply of crystals to recruit the exhausted strength 

 of this battery. 



Fig. 12. 



Having thus described briefly, but we trust, intelligibly, the 

 various Voltaic Instruments in use ; it may be requisite, before 

 dismissing them completely from notice, to make a few remarks 

 on some of the practical points attending their manipulation. 

 In the first place, it is absolutely necessary that the various con- 

 nections of batteries should be perfectly clean to insure a good 

 metallic contact. If this point be neglected, it will frequently 

 be found, that either the apparatus will not act at all, or at all 

 events be much diminished in power. The interior of the 

 binding screws, or the surfaces where the excited metals are, as 

 in Grove's battery, placed in contact with each other and the 

 ends of the conducting wires should be perfectly freed from 

 metallic oxides or dirt, by means of sand paper a very little 

 trouble being sufficient to effect this. The necessity of perfect 



