force. If the circuit is fixed and the magnet movable, then the force acting on the magnet will also be such as to tend to make the number of lines of force that pass through the circuit a maximum (see also Art. 317). 194. De la Rive's Floating Battery.-The preceding remarks may be illustrated experimentally by the aid of a little floating battery. A plate of zinc and one of copper (see Fig. 87) are fixed side by side in a large cork, and connected above by a coil of covered copper wire bent into a ring. This is floated upon a dish containing dilute sulphuric acid. If one pole of a bar magnet be held towards the ring it will be attracted or repelled according to the pole employed. The floating circuit will behave like the floating magnet in Fig. 44, except that here we have what is equivalent to a floating magnetic shell, If the S. pole of the magnet be presented to that face of the ring which acts as a S.-seeking pole (viz. that face round which the current is flowing If the pole be in a clockwise direction), it will repel it. thrust right into the ring, and then held still, the battery will be strongly repelled, will draw itself off, float away, turn round so as to present toward the S. pole of the magnet its N.-seeking face, will then be attracted up, and will thread itself on to the magnet up to the middle, in which position as many magnetic lines of force as possible cross the area of the ring. It can be shown also that two circuits traversed by currents attract and repel one another just as two magnetic shells would do. It will be explained in Lesson XXVI. on Electromagnets how a piece of iron or steel can be magnetised by causing a current to flow in a spiral wire round it. 195. Strength of the Current in Magnetic Measure. When a current thus acts on a magnet pole near it, the force ƒ which it exerts will be proportional to the strength of the current, and proportional also to the strength m of the magnet pole, and to the length 7 of the wire employed: it will also vary inversely as the square of the distance from the circuit to the magnet pole. Or, f= dynes. Suppose the wire looped up into a circle round the magnet pole, then l=2πr, and fƒ= m dynes. Suppose also that the circle is of one centimetre radius, and that the magnet pole is of strength of one unit (see Art. 125), then the 2πέ force exerted by the current of strength i will be X I, 2πί r i.l.m I I or 2πi dynes. In order, therefore, that a current of strength i should exert a force of i dynes on the unit pole, one must consider the current as travelling round only 2π part of the circle, or round a portion of the circumference equal in length to the radius. 196. Unit of Current Strength. -A current is said to have a strength of one "absolute" unit when it is such that if one centimetre length of the circuit is bent into an arc of one centimetre radius, the current in it exerts a force of one dyne on a magnet-pole of unit strength placed at the centre of the arc. The practical unit of "one ampère" is only roof this theoretical unit. (See also Art. 323.) LESSON XVII.-Galvanometers. 66 2:1 197. The term Galvanometer is applied to an instrument for measuring the strength of electric currents by means of the deflection of a magnetic needle, round which the current is caused to flow through a coil of wire. The simple arrangement described in Art. 188 was termed a Galvanoscope," or current indicator, but it could not rightly be termed a "galvanometer or current measurer, because its indications were only qualitative, not quantitative. The indications of the needle did not afford accurate knowledge as to the exact strength of current flowing through the instrument. A good galvanometer must fulfil the essential condition that its readings shall really measure the strength of the current in some certain way. It should also be sufficiently sensitive for the currents that are to be measured to affect it. The galvanometer adapted for measuring very small currents (say a current of only one or two millionth parts of an ampère) will not be suitable for measuring very strong currents, such as are used in producing an electric light. Moreover, if the current to be measured has already passed through a circuit of great resistance (as, for example, some miles of telegraph wire), a galvanometer whose coil is a short one, consist 1 The terms "Rheoscope" and "Rheometer" are still occasionally applied to these instruments. A current interrupter is sometimes called a "Rheotome," and the Commutator or Current Reverser, shown in Fig. 149, is in some books called a Rheotrope; but these terms are dropping out of use. M ing only of a few turns of wire, will be of no use, and a long-coil galvanometer must be employed with many turns of wire round the needle. The reason of this is explained hereafter (Art. 352). Hence it will be seen that different styles of instrument are needed for different kinds of work; but of all the requisites are that they should afford quantitative measurements, and that they should be sufficiently sensitive for the current that is to be measured. 198. Nobili's Astatic Galvanometer. — The instrument constructed by Nobili, consisting of an astatic pair of needles delicately hung, so that the lower one lay within a coil of wire The wound upon an ivory the needles; and three screw feet serve to adjust the base of the instrument level. Protection against currents of air is afforded by a glass shade. When a current is sent through the wire coils the needles move to right or left over a graduated circle. When the deflections are small (i.e. less than 10° or 15°), they are very nearly proportional to the strength of the currents that produce them. Thus, if a current produces a deflection of 6° it is known to be approximately three times as strong as a current which only turns the needle through 20. But this approximate proportion ceases to be true if the deflection is more than 15° or 20°; for then the needle is not acted upon so advantageously by the current, since the poles are no longer within the coils, but are protruding at the side, and, moreover, the needle being oblique to the force acting on it, part only of the force is turning it against the directive force of the fibre ; the other part of the force is uselessly pulling or pushing the needle along its length. It is, however, possible to “calibrate” the galvanometer,—that is, to ascertain by special measurements, or by comparison with a standard instrument, to what strengths of current particular amounts of deflection correspond. Thus, suppose it once known that a deflection of 32° on a particular galvanometer is produced by a current of Too of an ampère, then a current of that strength will always produce on that instrument the same deflection, unless from any accident the torsion force or the intensity of the magnetic field is altered. It is not 199. The Tangent Galvanometer. for the reasons mentioned above-possible to construct a galvanometer in which the angle (as measured in degrees of arc) through which the needle is deflected is proportional throughout its whole range to the strength of the current. But it is possible to construct a very simple galvanometer in which the tangent1 of the angle of deflection shall be accurately proportional to the strength of the current. Fig. 89 shows a frequent form of Tangent Galvanometer. The coil of this instrument consists of a simple circle of stout copper wire from ten to fifteen inches in diameter. At the centre is delicately suspended a magnetised steel needle not exceeding one inch in length, and usually furnished with a light index of aluminium. The instrument is adjusted 1 See note on Ways of Reckoning Angles, p. 109. |