VARIOUS VOLTAIC CELLS. =49. Single-Fluid and Two-Fluid Cells.= The simple cell (§ 45) is a single-fluid cell. The liquid is called the _electrolyte_, and this must act upon one of the plates; that is, chemical action must take place in order to produce a current. The simple cell polarizes rapidly, so something must be used with the dilute sulphuric acid to destroy the hydrogen bubbles. This is done in the _bichromate of potash cell_. In order to get complete depolarization--that is, to keep the carbon plate almost perfectly free from hydrogen, it is necessary to use _two-fluid cells_, or those to which some solid depolarizer is added to the one fluid. =50. Open and Closed Circuit Cells.= If we consider a voltaic cell, the wires attached to it, and perhaps some instrument through which the current passes, we have an _electric circuit_. When the current passes, the circuit is _closed_, but when the wire is cut, or in any way disconnected so that the current can not pass, the circuit is _open_ or _broken_. (See "Study," § 266.) _Open Circuit Cells_ are those which can give momentary currents at intervals, such as are needed for bells, telephones, etc. These must have plenty of time to rest, as they polarize when the circuit is closed for a long time. The _Leclanché_ and _dry_ cells are the most common open circuit cells. _Closed Circuit Cells._ For telegraph lines, motors, etc., where a current is needed for some time, the cell must be of such a nature that it will not polarize quickly; it must give a strong and constant current. The _bichromate_ and _gravity cells_ are examples of this variety. (See "Study," § 286.) =51. Bichromate of Potash Cells= are very useful for general laboratory work. They are especially useful for operating induction coils, small motors, small incandescent lamps, for heating platinum wires, etc. These cells have an E.M.F. of about 2 volts. Dilute sulphuric acid is used as the exciting fluid, and in this is dissolved the bichromate of potash which keeps the hydrogen bubbles from the carbon plate. (See "Apparatus Book," § 26.) Zinc and carbon are used for the plates, the + pole being the wire attached to the carbon. [Illustration: Fig. 39.] Fig. 39 shows one form of bichromate cell. It furnishes a large quantity of current, and as the zinc can be raised from the fluid, it may be kept charged ready for use for many months, and can be set in action any time when required by lowering the zinc into the liquid. Two of these cells will burn a one candle-power miniature incandescent lamp several hours. The carbon is indestructible. =Note.= For various forms of home-made cells, see "Apparatus Book," Chapter I., and for battery fluids see Chapter II. =52. The Grenet Cell.= Fig. 40 is another form of bichromate cell. The carbon plates are left in the fluid constantly. The zinc plate should be raised when the cell is not in use, to keep it from being uselessly dissolved. [Illustration: Fig. 40.]