1. What is meant by an Electric Current, and by its Direction of Flow -2. Properties of an Electric Current-3. Measuring the Strength of a Current-4. Conductors and Insulators-5. The Strength of an Electric Current: by which of its Properties shall it be Directly Measured ?-6. Definition of the Unit Current; Ampere -7. Definition of the Direction of the Current-8. Objection to the Usual Mode of Constructing Voltameters-9. Description of Practical Forms of Sulphuric Acid Voltameters-10. Relative Advantages of Voltameters and Galvanometers-11. Meaning of the Relative and the Absolute Calibration of a Galvanometer12. Experiment for Calibrating a Galvanometer Relatively or Absolutely-13. Graphically Recording the Results of an Experiment-14. Practical Value of Drawing Curves to Graphically Record the Results of Experiments-15. To Construct a Galvanometer Scale from which the Relative Strengths of Currents can be at once Ascertained. 1. What is meant by an Electric Current, and by its Direction of Flow.--In the various industries in which electricity is employed, as in the telegraph, telephone, electric lighting, electrotyping, electroplating, torpedo exploding, electric traction, the electric transmission of power, and in the working of machinery by the aid of electromotors, it is the so-called "electric current" that is made use of. Hence a knowledge of the laws of this electric current, a clear conception of its so-called properties, combined with a practical acquaintance with the modes of measuring it, must be of especial importance for a right understanding of the working of the apparatus employed in the above-mentioned B industries. Indeed, such knowledge is absolutely necessary if the user of electrical apparatus is desirous of employing it to the best advantage, of being able to correct faults when they occur, as well as of effecting improvements in the instruments themselves. It is customary to speak of an electric current as if it had an independent existence apart from the " conductor" through which it is said to be flowing, just as a current of water is correctly spoken of as something quite distinct from the pipe through which it flows. But in reality we are sure neither of the direction of flow of an electric current, nor whether there is any motion of anything at all. And the student must not assume that the conventional expression, "The current flows from the copper pole of a galvanic battery to the zinc pole through the external circuit," implies any knowledge of the real direction of flow any more than the railway expressions, "up train" and "down train," mean that either train is necessarily going to a higher level than the other. In the case of a stream of water flowing along a river-bed we are quite certain that there is water in motion, and everyone is agreed as to which way the water is flowing; a cork or a piece of wood thrown on the water indicates by its motion the direction in which the water is moving. Nor, again, must an electric current be supposed to be like waves of sound travelling along, since in this latter case, although there is no actual travelling along of matter, still the direction of motion of the wave of sound is perfectly definite. Indeed, a wire along which an electric current is flowing is more like a wire at each end of which a musical instrument is being played, so that the sound is travelling in both directions along the wire at the same time. In short, the statement that an electric current is flowing along a wire is only a short way of expressing the fact that the wire and the space around the wire are in a different state from that in which they are when no electric current is said to be flowing. So that when a body and the space around the body possess certain properties that they do not usually possess, an electric current is said to be flowing through that body. 2. Properties of an Electric Current. These properties are: (1) A suspended magnet put in nearly any position near a body through which an electric current is said to be flowing will be deflected, showing that a force is exerted on the magnet (Fig. 1). This force is mutual, so I Fig. 1. that if a magnet be brought near any substance traversed by an electrical current, this substance will generally be acted upon by a force tending to move it (Fig. 2). Also any piece of soft iron put near a conductor carrying a current will become magnetised (Fig. 3). The action in all these cases is just as if the body conveying the current had become magnetic. This is further shown by the fact that any two wires through each of which a current of electricity is passing, act upon each other with a magnetic force in nearly every position in which the wires may be placed relatively to one another (Fig. 4). (2) If the circuit through which an electric current is said to be flowing be partly solid and partly liquid, Fig. 2.-A Piece of Tinsel Coiling itself round a Magnet when a Current Flows through the Tinsel. Suggested by Prof. Jamieson. then the liquid will generally be decomposed into two parts, one part going to one side of the liquid in the direction in which the current may be said to be flowing, and the other part going to the other side of the liquid in the opposite direction to the flow of the current (Fig. 5). (3) The body conveying the current becomes more or less heated (Fig. 6). In popular language the current is said: (1) To deflect the magnet, and magnetise the iron. (3) To heat the body through which it is flowing. Fig. 3.-Iron Rod Picking up Nails when a Current Flows through a Wire Coiled round it. Wire may be Bare if Rod be Varnished. But as we have no evidence of the current apart from the conductor through which it is said to flow, it is more accurate to speak of a current being said to flow through a conductor in which these effects are found to be produced, than to say that the current produces these effects. The latter expression, however, for brevity's sake, is generally adopted; and, indeed, the heat generated in a wire conveying a current has so many analogies with the heat produced in a pipe by the |