DEFINITIONS RELATING TO PRIMARY BATTERIES.

The Voltaic-cell is very commonly called the voltaic battery after its discoverer, Volta, and more recently named the primary battery, to distinguish it from the secondary or storage battery.

A cell consists of a vessel containing a liquid in which two elements are immersed. In One Fluid cells both electrodes are immersed in the same solution. In Two Fluid cells each electrode is immersed in a separate solution, one of which is contained in a porous cup which is then immersed in the other liquid.

Dry cells are similar in construction to the open circuit, one fluid cell. The only difference being that starch or some other absorbant is mixed with the liquid forming a jelly which is not easily spilled. The dry cell is very efficient, but when once exhausted is of no further use.

All dry cells should have vent holes.

The Electrodes of a Primary Battery are the plates of metal or other substance immersed in the liquid. The zinc plate is called the generating electrode and the other plate the conducting electro. The latter is usually made of carbon, copper being the next most commonly used.

NOTE. The plates in a galvanic couple are termed elements as the carbon and zinc plates in cell. The plate unattacked by the solution as the carbon plate in the above battery is termed the negative plate or element; the one attacked, as the zinc plate, is termed the positive plate or element.

DEFINITIONS-PRIMARY BATTERIES.

The Poles of a battery are the parts of the Electrodes which project out of the liquid. They are distinguished from each other by + for the positive and for the negative pole. The terms Pole or Terminal apply to the ends of a break in any electric circuit.

The Exciting Fluid is the liquid which when plates are placed in it acts upon the plates and produces a current in a wire joining the two plates.

The electrolyte is another term for the exciting fluid.

The Anode is the plate which the current leaves to enter the liquid of the cell as it flows through the circuit of that battery.

The Kathode (or Cathode) is the plate the current enters as it leaves the liquid of the cell as it flows through the circuit of that battery.

Polarization is the weakening of the battery current by means of local action; this is commonly caused by the collec tion of hydrogen bubbles on the copper plate. Polarization is overcome in two ways, namely, chemically and mechanically. In the first mentioned a solution or substance which will absorb the free hydrogen is introduced in the cell. In the second the plates have a roughened surface and are kept moving in the solution.

Local Action.-When the circuit is not closed the current cannot flow, and there should be no chemical action so long as the battery is producing no current. The impure zinc of commerce, however, does not remain quiescent in the acid,

DEFINITIONS-PRIMARY BATTERIES.

but is continually dissolving and giving off hydrogen bubbles causing local action. The impurities in the zinc consists of particles of iron, arsenic, and other metals.

Separating the elements.-Obviously the positive and nega. tive elements of a battery must not be in contact within the exciting fluid; they should be separated by a space of 3% to 1⁄2 inch. In the case of batteries without porous cells, periodical attention will need to be given to ensure this condition being maintained.

Electrolysis is the decomposition of a chemical compound by the electric current.

Bichromate Batteries of bottle shape as in Fig. 20, with two carbon plates, a sliding rod and movable zinc plate, are very extensively used by experimenters and lecturers, because they are always ready for being put to work with one motion of the hand, not necessitating any other preparation; and as soon as the desired result is obtained, the battery can be put out of action with the same facility.

MANAGEMENT AND CARE OF BATTERIES.

Cleanliness in the battery-room is essential to the best results. The jars, before being used, should be coated with parafine wax for an inch or so from the top. This prevents "creeping" of the salts and consequent weakening of the battery fluid. Zincs and coppers, or their homologous elements, should be thoroughly cleaned every time the cell is taken out of use. The zinc, after being thoroughly cleaned, should be rubbed with a little mercury. This prevents local action. Porous cups should be soaked in clean water four or five hours and then wiped dry. If the cells are cleaned and put away ready for use, when the emergency does come, the time spent in cleaning will never be regretted.

THE MAGNETIC AND ELECTRIC CURRENT.

This branch of science comes naturally under the head of Current Electricity, but,

To give a definition of the electric current is like attempting to describe something which every one knows about; it can be said to be simply the flow of electric energy from a point of high pressure to that of a lower, yet it must be owned that until we know with absolute certainty what electricity is we cannot expect to know precisely what a current of electricity is.

NOTE-The circuit may be compared to a system of hot-water pipes; in a hot water system there is a steam boiler, a flow and a return circulating pipe with pipe coils, at various points for giving off heat where warmth is required.

In an electric installation the dyamo or battery replaces the boiler; flow and return pipes are represented by the two conducting mairs and the pipe-coils by lamps, motors and other apparatus.

In a hot-water system it is perfectly evident that whatever may be the quantity of water leaving the boiler, the same must return to it and the quantity of water passing through every part of the flow and return pipes in a given time must be alike.

It is so ith the electric current; whatever may be the quantity of the current starting from the dynamo the same quantity comes back to it.

Also, the whole of this quantity must pass through the mains, but it is different with the pressure; this diminishes in proportion to the work done by the current. Consequently the pressure diminishes as the current advances along its path.

Comparing this again with the hot-water system, the analogy is fairly complete; for the current may be regarded as losing heat (this is often disguised) as it advances, since in losing its pressure it produces heat and the hot water does the same.