gilt paper and so-called silver paper, placing them in pairs, the gilt face of one against the silver face of the other, and then making a pile of such pairs, the same kind of paper being uppermost in each pair. In a moist climate the natural dampness of the paper enables it to play the part of the acidulated cloth layers in Volta's pile.

595. Voltaic Cell.-A cell having a plate of zinc and a plate of copper dipping in dilute sulphuric acid is known as a simple Voltaic cell, and several cells combined constitute a Voltaic or Galvanic battery. Since Volta's day many improved kinds of battery cells have been devised, some of which will be considered later (§§627-637).

The two plates of a Voltaic cell are called the electrodes, and the terminals of the plates where the external wires are connected are called the poles of the cell. The copper terminal is at a higher potential than the zinc terminal and gives a positive charge, it is therefore called the positive pole, while the zinc terminal is the negative pole.

On the other hand, the copper plate is often spoken of as the negative electrode or electronegative element in the cell and the zinc as the positive electrode or electropositive element, because positive charge is transmitted through the acid of the cell from the zinc to the copper plate as though repelled by the zinc and attracted by the copper.

Cu

Zn

Electric Current in a Cell.-Since the two poles of a Voltaic cell are at different potentials, an electric current is established when they are connected by a metallic wire just as when the two coats of a charged Leyden jar are connected. This current, however, flows steadily instead of lasting only for an instant. The metallic wire together with the plates and liquid between them form a conducting circuit in which the positive direction of the current or that in which positive electricity flows is from copper to zinc through the outside wire and from zinc back to copper inside the liquid of the cell.

FIG. 337.-Electric circuit.

There are three principal evidences of the existence of the

current:

1. Heat is developed in all parts of the circuit.

2. Every part of the circuit affects a magnetic needle brought near it.

3. Chemical action takes place at the surfaces of contact between the metal electrodes and the liquid. If the copper and zinc plates are in dilute sulphuric acid, bubbles of hydrogen gas appear at the surface of the copper plate, while the zinc plate is eaten away by the acid, and zinc sulphate is formed.

All these phenomena cease at once when the current is interrupted, either by breaking the metallic connection between the plates or by separating the acid around one plate from that around the other by a non-conducting partition.

596. Contact Potentials in a Voltaic Cell.-When zinc is immersed in the acid there is what may be called a solution. pressure, or tendency for the zinc to be dissolved and form zine sulphate in solution, each atom of zinc carrying into the solution. a positive charge.

As the positively charged atoms of zinc pass into solution, the plate, losing positive charge with each one, becomes negative, while the solution becomes positive, in consequence of which there is an electrostatic force tending to prevent the positively charged zinc atoms from going into solution. Therefore when a certain difference of potential between the zinc and acid solution is reached there will be equilibrium between the electrostatic force and the solution tendency, and the zinc will cease to be dissolved.

There is thus a definite difference of potential due to the contact of zinc and acid when there is equilibrium between them, and another due to the contact of copper and acid which is less than the former since the solution pressure of copper in the acid is less than that of zinc.

597. Electromotive Force.-The diagram, figure 338, represents the relative potentials of the elements in a Voltaic cell. The acid has the highest potential and is positive both to zinc and copper. The difference between acid and copper is, however, less than between it and zinc, and the copper is therefore at a higher potential than the zinc as shown.

If the copper pole of the cell is connected to the earth, it comes to the earth potential or zero, and the zinc pole as tested by a quadrant electrometer is found to have a negative potential.

On the other hand if the zinc pole is connected to the earth it will be at zero potential while the copper pole will be found to be positive; but the difference of potential between them will be the same in each case.

Every cell can produce a certain maximum difference in potential between its two electrodes, and when this is reached. there is equilibrium and the chemical action stops.

The power of a cell to produce difference of potential is called its electromotive force; it is measured by the difference of potential between the electrodes when there is

no current and the chemical action has ceased.

The electromotive force of a cell depends only on the chemical relations of the constituents of the cell and is there

Liquid Potential

Copper Potential

Zinc Potential

FIG. 338.

fore the same whether the plates are large or small.

A small cell formed by dipping the tips of a zinc and of a copper wire into a single drop of acid will cause as great a deflection of a quadrant electrometer as a cell of the same kind with plates a foot square.

A convenient abbreviation for electromotive force is E. M. F., or in equations the symbol E is commonly used.

598. Unit of Electromotive Force.-The unit of electromotive force used in practice is the volt, named in honor of Volta. It is much smaller than the electrostatic unit of potential defined in §528, the latter being almost exactly equal to 300 volts. The definition by which the exact value of the volt is fixed will be given later ($717).

The Voltaic cell has an electromotive force of nearly one volt. 599. Hydraulic Analogy to Voltaic Cell. The following analogy given by Lodge is instructive. Two tall open vessels containing water are connected by a pipe in which is a pump P driven by a weight W (Fig. 339). The water will flow from one vessel to the other until the back pressure on the pump due to the higher level of B just balances the force of the weight. The difference in level will be the same whether the vessels are large or small. The difference of level represents the difference of potential between the zinc and copper which is independent of the size of the cell, the pump with its driving weight is the

electromotive force of the cell, which through chemical action can produce a certain definite difference of potential and no more.

Figure 340 represents the state of things when the zinc and copper plates are connected by a wire, represented by the tube shown. The difference of pressure causes a flow through the tube from B to A, at the same time the level sinks in B and rises in A so that the difference in pressure on the two sides diminishes and is no longer able to balance the pressure of the pump, which therefore begins to act, forcing water from A to B; at the same time the weight W descends, supplying energy for the circulation, which will be maintained so long as the weight can move downward.

Here it is seen that the electromotive force, represented by the power of the pump to produce pressure, is the same as before, but the difference of potential between the plates, shown by the difference between the levels of A and B is less than before. The work done by the pump in circulating the water is obtained from the weight, which loses potential energy as it descends. So in the Voltaic cell, the energy expended by the electric current is supplied by the chemical changes which take place at the electrodes.

600. Magnetic Effect of Current.-In 1819 Oersted discovered that when a wire connecting the poles of a Voltaic cell was held over a balanced magnetic needle and parallel to it, the needle was deflected, the north pole of the needle moving toward the west when the current was from south to north, as in the diagram, while if the current was reversed the north pole of the

needle moved toward the east. The effect was reversed when the wire was placed under the needle.

This discovery aroused the greatest interest, as it was the first evidence of a connection between magnetism and electricity.

601. Electric Circuit.-It was also found that the action was the same whatever part of the wire connecting the plates was brought near the needle, the deflection produced by the current in the middle of the wire being just as great as that near its ends.

Copper

S

Zino

FIG. 341.-Current and magnetic needle.

By this experiment also the direction of the current in the electrolyte may be shown to be opposite to that in the wire; for if two vessels are used connected by a short tube containing the acid, and if a zinc plate is placed in one vessel and copper in the other, as shown in figure 342, a magnetic needle will be deflected toward the west when placed under the wire connecting the plates, but toward the east when under the tube. The experiment shows that the current in the electrolyte is just as strong as that in the wire, but in the opposite direction.

Cu

S

N

Zn

From experiments such as the above it is inferred that steady electric currents always flow in closed circuits and are equally strong at every point, and if the circuit is interrupted at any point, whether in the electrolyte or the wire, the magnetic action. and all other current effects cease everywhere at almost the same instant. It is very much as when an incompressible liquid circulates in a closed tube, just as much liquid must pass any one section of the tube as any other during the same time.

N

FIG. 342.-Current in electrolyte.

602. Galvanometers. When a wire is bent into a vertical circle having its plane parallel to the direction of a magnetic needle balanced at its center, if a current is established in the wire all parts of it act together to deflect the needle, turning the north pole to one side or the other, depending on the direction of the current. An instrument which measures electric