(B) Replace the plates in the cell, and connect them with the 15-turn coil of G V, placed N and S. Allow the circuit to remain closed for half an hour, and record the position of the needle every 5 minutes. (C) Again wash, dry with a cloth without rubbing, and weigh both the zinc and copper. Compare the new weights with those found in (A). _=285. Chemical Action in the Two-Fluid Cell.=_ In this form of cell the hydrogen is not allowed to collect upon the copper plate. The action inside of P C is like that explained in § 270, hydrogen being set free. As soon as this hydrogen (H_{2}) comes in contact with the copper sulphate (CuSO_{4}), and it begins to do this in the walls of P C, a new chemical reaction takes place. Hydrogen has a stronger attraction for SO_{4} than Cu has, so it unites with the SO_{4}, forming H_{2}SO_{4} (sulphuric acid), and this at the same time throws out the Cu bodily. This Cu is then free, instead of hydrogen, to be deposited upon the copper plate. The current is constant, as there is no polarization. _=286. Various Galvanic Cells; Open and Closed Circuit Cells.=_ There is no one form of cell that is best for all kinds of work. If momentary currents only are wanted, such as for bells, telephones, etc., in which the cell has plenty of time to rest, _open circuit_ cells are used. These cells polarize, however, when the circuit has to be closed for any length of time. This form of cell is always ready for use, and may not need attention for months at a time. The most common forms of the open circuit cells are the _Leclanché_ (§ 287) and _dry_ cell (§ 288). Open circuit cells polarize quickly, because the depolarizer (§ 280) is slow in destroying the hydrogen. When a strong current is needed for a considerable time, such as for telegraph lines, motors, etc., a _closed circuit_ cell is necessary. The depolarization must be rapid and constant. The _bichromate_ (§ 289) and the _Daniell_ cell (§ 290) are very common forms of closed circuit cells. (See, also, Storage Cells.) =287. The Leclanché Cell= is an open circuit cell in which carbon and zinc are the plates. The carbon is surrounded with dioxide of manganese, a depolarizer; the two are either packed together in a porous cup, or the latter is compressed into blocks, which are fastened to the carbon. The porous cup stands in a jar containing a solution of sal ammoniac (ammonium chloride), which acts as the exciting fluid, and in which stands a zinc rod. The zinc is not acted upon when the circuit is open. The hydrogen given off by the decomposition of the ammonium chloride is destroyed by the oxygen contained in the manganese dioxide. The E. M. F. is nearly 1.5 volts.