Imágenes de páginas
PDF
EPUB

A dry pile, built up on the general plan of Volta's moist pile, was devised by De Luc, and improved by Zamboni. In Zamboni's construction, sheets of paper are prepared by pasting finely laminated zinc or tin on one side, and rubbing black oxide of manganese on the other. Discs are punched out of this paper, and piled up into a column, with their similar sides all facing the same way, to the number of a thousand or upwards, and are well pressed together. The difference of potential between the two ends is sufficient to produce sensible divergence of the gold-leaves of an electroscope, but the quantity of electricity which can be developed in a given time is exceedingly small. No pile or battery can generate a sensible current, except by a sensible consumption of its materials in the shape of chemical action.

A very delicate gold-leaf electroscope was devised by Bohnenberger, consisting of a single leaf suspended between the two poles of a dry pile, which for this purpose is arranged in two columns connected below, so that the poles are at the summits. If their lower ends, which form the middle of the series, be connected with the earth, one pole will always have positive, and the other negative. potential. A very slight charge, positive or negative, given to the gold-leaf by means of the knob at the top of the case, suffices to make it move to the negative or the positive pole. 708A. Bichromate Battery.-The most convenient cells for most

class experiments are the bichromate of potash bottle-cells, one of which is represented in Fig. 458A. The liquid is a solution of bichromate of potash, with a little sulphuric acid added. In this liquid two flat plates of carbon are suspended, and between them is a flat plate of zinc, which can be slid up and down by means of a rod projecting through the top of the cell. It is slid up when not in use, and is then just clear of the liquid. By pushing it down (which can be done instantaneously), the cell is brought into full action, and as soon as the experiment is concluded the zinc should again be raised out of the liquid. The cell is not suited for long-continued work, but it gives powerful effects when only used for a few minutes at a time. It has the conveniences of great portability and of freedom from noxious fumes.

Fig. 458 A.-Bichromate

Bottle-cell.

CHAPTER LIV.

GALVANOMETER.

709. Ersted's Experiment.-The discovery by the Danish philosopher Ersted, in 1819, that a magnetized needle could be deflected by an electric current, was justly regarded with intense interest. by the scientific world, as affording the first indication of a definite relation existing between magnetism and electricity.

Ersted's experiment can be repeated by means of the apparatus represented in Fig. 459. Two insulated metallic wires are placed in the magnetic meridian, one of them above, and the other below a

[subsumed][merged small][merged small][graphic]

instead of the upper wire. It will also be reversed by reversing the direction of the current. In the figure, the current is supposed to be passing above the needle from south to north. In this case the north end of the needle moves to the west, and the south end to the east. On making the current pass in various directions, either horizontally, vertically, or obliquely, near one pole of the needle, it will be found that deviation is always produced except when the plane containing the pole and current is perpendicular to the length of the needle.

710. Ampère's Rule.-The direction in which either pole of a needle is deflected by a current, whatever their relative positions may be, is given by the following rule, which was first laid down by Ampère.

[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Imagine an observer to be so placed that the current passes through him, entering at his feet and leaving at his head, then the deflection of a north-seeking pole will be to his left. The deflection of a southseeking pole will be in the opposite direction. The two figures 460, 461 illustrate the application of this rule to the two cases just considered. The current is supposed, in both cases, to be flowing from south to north. A is the austral or north-seeking pole of the needle, and B the boreal or south-seeking pole.

C

711. Lines of Magnetic Force due to Current.-The relation between currents and magnetic forces may be more precisely expressed by saying that a current flowing through a straight wire produces circular lines of force, having the wire for their common axis. A pole of a magnet placed anywhere in the neighbourhood of the wire, experiences a force tending to urge it in a circular path round the wire, and the direction of motion round the wire is opposite for opposite poles. Fig. 462 represents three of the lines of force for a north-seeking pole, due to a current flowing through a straight wire from the end marked + to the end marked. The lines of force are circles (shown in perspective as ellipses), having their centre at a point C in the wire, and having their plane perpendicular to the length of the wire. The arrows indicate the direction in which a

+

Fig. 162.-Lines of Force due to Current.

REACTION OF MAGNET.

699

north-seeking pole will be urged. This direction is from right to left round the wire as seen from the wire itself by a person with his feet towards and his head towards -, according to Ampère's rule. The figure may be turned upside down, or into any other position, and will still remain true.

+

712. Reaction of Magnet on Current.-While the wire, in virtue of the current flowing up through it, urges an austral pole from A

+

Fig. 46. Reaction on
Current.

towards A' (Fig. 463), it is itself urged in the opposite direction C C'. If an observer be in imagination identified with the wire, the current being supposed, as in Ampère's rule, to enter at his feet, and come out at his head, the force which he will experience from a north-seeking pole directly in front of him will be a force to his right. It will be noted that the magnetic influence which thus urges him to the right, would urge a north-seeking pole from his front to his back. A conductor conveying a current is not urged along lines of magnetic force, but in a direction which is at right angles to them, and at the same time at right angles to its own length.

713. Numerical Estimate of Currents.-The numerical measure of a current denotes the quantity of electricity which flows across a section of it in unit time. It is sometimes called strength of current, sometimes, especially by French writers, intensity of current, sometimes simply current or amount of current. If a thin and a thick wire are joined end to end, it has the same value for them both; just as the same quantity of water flows through the broad as through the contracted parts of the bed of a stream. Hence the name intensity is obviously inappropriate, for, with the same total quantity of electricity flowing through both, the current is, properly speaking, more intense in the thin than in the thick wire.

Currents may be measured experimentally by various tests, which are found to agree precisely. The most convenient of these for general purposes is the deflection of a magnetized needle. The force which a given pole experiences in a given position with respect to a wire conveying a current, is simply proportional to the current. Hence the name strength of current admits of being interpreted in a

sense corresponding to that in which we speak of the strength of a pole. Instruments for measuring currents by means of the deflections which they produce in a magnetized needle are called galvanometers.

[graphic]

714. Sine Galvanometer.

-The sine galvanometer, which was invented by Pouillet, is represented in Fig. 464. The current which is to be measured traverses a copper wire, wrapped round with silk for insulation, which is carried either once or several times round a vertical circle; and this circle can be turned into any position in azimuth, the amount of turning being indicated on a horizontal circle. In the centre of the vertical circle, a declination needle is mounted, surrounded by a horizontal circle for indicating its position, this circle being rigidly attached to the ver

Fig. 464.-Sine Galvanometer.

tical circle. Suppose that, before the current is allowed to pass, both the needle and the vertical circle are in the magnetic meridian, and that the needle consequently points at zero on its horizontal circle. On the current passing, the needle will move away. The vertical circle must then be turned until it overtakes the needle; that is, until the needle again points at zero. This implies turning the circles through an angle a equal to that by which the needle finally deviates from the magnetic meridian. In this position the terrestrial couple tending to bring back the needle to the meridian is proportional to sin a (§ 679). The forces exerted upon the two poles by the current are perpendicular to the plane of the vertical circle, and are simply proportional to the current. Hence, in comparing different observations made with the same instrument, the amounts of current are proportional to the sines of the deviations.

« AnteriorContinuar »