as the hydrogen separates, it acts at once on the copper sulphate, depositing copper on the positive electrode.

In the Grove cell we have in the outer cup zinc and dilute sulphuric acid, and in the inner porous cup platinum and nitric acid. In the Bunsen cell we have gas carbon substituted for platinum in the inner cup, while zinc constitutes the outer element.

In the Leclanché cell, shown in Fig. 29, the inner porous cup contains a carbon plate packed in fragments of coke and powdered manganese dioxide, while the rod of zinc dips into a solution of sal ammoniac in the outer cup.

The porous cup is dispensed with in what are called "gravity" batteries, where a liquid of superior gravity is placed below and

a lighter one is run in on top. Thus, in the Callaud cell we have a saturated solution of sulphate of copper below, with the copper plate resting on the bottom of the cup and connected with the circuit by an insulated wire, and above this zinc and dilute sulphuric acid, or, as it soon becomes, zinc sulphate solution.

Of one-liquid cells still in use, the most popular is the bichromate dipping battery, shown in Fig. 30. In this the zinc plate can be raised out of the liquid when not in use, and so the action stopped; the polarization of the positive electrode is, moreover, prevented by chemical action, as the solution is bichromate of potash acidified by sulphuric acid, and the escaping hydrogen reduces the chromic acid in the degree that it is formed. Of course, when the chromic acid is all reduced the solution has to be renewed.

A number of voltaic cells may be coupled together so as to

increase the strength of the current, and then we have a voltaic battery. The coupling may be effected in two ways: each zinc may be coupled to the copper or carbon of the next cell, and so on to the end of the line, in which case the battery is said to be arranged in series, or the zincs may all be coupled together and the carbons all together, in which case the battery is said to be arranged in multiple or in parallel circuit.

2. Electrical Units.-When electricity is passing, even though it be through what we call a conductor, it meets with resistance. For purpose of comparison it is desirable to have a uniform method of measuring this. The unit of resistance is called the ohm, and is the resistance of a column of pure mercury, having a section of one square millimeter and a length of 106.28 centimeters, at a temperature of o° C. For convenience, coils of wire with a known resistance in ohms are used. These are known as resistance coils and are prepared in sets, so that any resistance within quite wide limits can be measured with their aid. It is by such means that the location of a break in the ocean telegraph cables can be ascertained so that the cable may be grappled for and repaired.

The unit of current is called the ampère. We measure the amount of currents by noting the weight of copper that may be caused to separate from copper sulphate solution within a given time by the passage of the current. An ampère of current will deposit 0.327 milligramme of copper a second, or 1.177 grammes per hour. The current is also often measured by the amount of hydrogen and oxygen liberated within a given time by the electrolysis of water (see p. 97).

The ampère-meter is an instrument the dial face of which is graduated in ampères, while a needle deflected by the passage of the current through coils of wire moves over the scale and indicates the strength of the current.

The pressure or difference of potential which causes the electricity generated in a battery or cell to overcome the resistance of the circuit and so effect its passage is called its electro-motive force. The unit of electro-motive force is called the volt. It is the pressure required to maintain a current of one ampère through a resistance of an ohm. Electrometers are used for measuring difference of potential or electro-motive force.

The relation of these three units is concisely expressed in Ohm's law: The strength of the current is equal to the electro-motive force divided by the resistance.

The units ohm, ampère, and volt were named in honor of the three great electricians, Ohm, Ampère, and Volta.

3. Effects of Current Electricity. The main distinctions between the frictional current or discharge and the current developed by voltaic action are in tension or difference of potential and in amount of the current. Frictional electricity is of high tension but small in total amount, while the voltaic current is of low tension but greater in amount as measured in ampères. Hence the effects will differ somewhat. The mechanical effects of voltaic electricity are very slight as compared with those of frictional electricity. The physiological effects are also, as a rule, very much milder than those of the spark discharge. The wires from a voltaic battery of a dozen cells may be held in the hands without appreciable shock, but a tingling sensation is felt from very strong currents, and prolonged contact with the wires has an exciting effect upon the nerves. The heating effects are dependent, of course, upon the resistance which the circuit offers to the passage of the current and to the amount of current passing. With good conductors like copper, of sufficient section, the heating effect is slight; with wires of insufficient section, or with poor conductors, like platinum, iron, or German silver, the resistance becomes relatively great and the wire becomes strongly heated. It is thus possible, with relatively moderate currents, to fuse a thin platinum wire which may be interposed in the circuit. The luminous effects of voltaic electricity are obtained under two distinct conditions: First, when two wires from the electrodes of a battery are brought together, thus closing the circuit, a spark is seen at the point of contact, often of great brilliancy. This is also seen on breaking the circuit. Secondly, the resistance offered to the passage of the current through a poor conductor often heats the latter to such a degree that it becomes luminous. Both of these methods of obtaining luminous effects, we will see later (see pp. 101, 102), are practically applied in electric lighting.

The chemical effects of the battery current are notably more important than are those of frictional electricity, because the greater amount of electricity and the duration of the current in the first case make possible effects not to be attained in the other case. Thus the current passed through acidified water, as shown in Fig. 31, will decompose it into its constituents, hydrogen and oxygen gases, by a process termed electrolysis. Similar results are obtained with solutions of many chemical salts.

This will be

more fully treated of later under the electro-deposition of the metals, or electro-metallurgy (see p. 102).

4. Electro-Magnetism.-The magnetic effects of the voltaic current are so important that they require special consideration. If a bar of soft iron is held at right angles to a wire carrying a current of electricity, we will find that it acquires for the time. being magnetic properties. If the wire be coiled around the bar, the magnetic effect will be increased in proportion to the number of coils. To make a powerful horse-shoe magnet, therefore, it is only necessary to take two short bars of soft iron joined at one end by a cross-piece of similar metal, surround these bars FIG. 32.

FIG. 31.

by coils or bobbins of insulated wire, and pass a current through the coils. While the current passes we have, as shown in Fig. 32, a powerful magnet, but, as soft iron has little or no coercive force, the moment the current ceases the magnetism of the iron cores disappears. The coils should be wound or connected so that the current passes around one coil in one direction and around the other in the opposite direction, in order that one shall form a north pole and the other a south pole. Electromagnets are used in almost all forms of practical electrical apparatus.

5. Voltaic Induction.-We have already spoken of the in

ducing action of a body charged with frictional electricity upon neutral bodies. This is sometimes called electro-static induction. In the case of voltaic electricity similar phenomena are very readily produced. If a magnet is moved near a wire, a current of electricity will be produced in the wire; if an electro-magnet have a current sent through its coils, a current will be produced in neighboring wires or coils. Such currents are called induced or secondary currents. They differ in many important respects from the primary currents which may be used to incite them. The chief difference is in the relation of electro-motive force or tension to volume of current. As before stated, the primary voltaic current is of low tension but relatively large volume. The secondary or induced current is more like the frictional electrical discharge, of high electro-motive force but relatively small volume. We shall refer in the next section to the practical application of this change in character of the voltaic current.

C. APPLICATIONS OF ELECTRICITY.

1. Of Electro-Magnetism.-The first and most important application of the principles of electro-magnetism is the magnetic telegraph, which was proposed in practical form by Prof. Morse in 1837. The Morse telegraph apparatus consists in the main of an electro-magnet, which, when a current passes through its coils, attracts an armature. An operator can cause this armature to move, even though many miles away, by simply sending a current over a wire leading to the station containing the apparatus. The essential instruments are the key, by means of which the sender controls the current and can establish or break contact by a pressure of the finger, and the sounder, or receiving instrument, which contains the electro-magnet, the movements. of whose armature cause a clicking sound. Sometimes, instead of the sounder, a register is substituted, in which case the armature causes marks to be made upon a long paper ribbon or tape. The alphabet of the Morse system consists of a series of dots and dashes upon the tape or of longer or shorter clicks as heard by the sounder. A relay or additional electro-magnet connected with a local battery is generally inserted in the case of long circuits, so as to strengthen the current and enable it to be transmitted. The wire at either end of the line is "grounded," so that the earth is made to act as return conductor, and thus a second wire is dispensed with.

Electric bells also involve an application of the electro-magnet.