Page:The American Cyclopædia (1879) Volume VII.djvu/605

 GALVANISM 593 ing force exerted upon the needle will be mul- tiplied nearly as many times as the wire passes backward and forward. Schweigger's multi- plier, constructed in this manner, is shown in fig. 14. The sensitive- ness of the instrument is increased by using what is called an asta- tic needle, which is constructed by placing two magnetic needles upon the same axis, but with their north and south ends in op- posite directions, and suspending them hori- zontally by a delicate fibre of silk. If their axes are perfectly parallel, and they have pre- cisely the same magnetic force, they will form a system which is astatic ; that is, they will when acted upon only by the earth's magnet- ism point indifferently in any direction. It is however impossible to place them perfectly FIG. 14. Schweigger's Multiplier. FIG. 15. Astatic Galvanometer. parallel, and it therefore follows that when they have equal magnetic force they will only come to rest when at right angles to the plane of the magnetic meridian. It is usual, how- ever, except in the most delicate tests, to have one of the needles slightly stronger than the other, so that there shall be a slight directive tendency north and south to the system. If a wire carrying a current is held between the two needles, they will both be deflected in the same direction ; and if the wire is formed into a coil, the force will be multiplied. An astatic galvanometer is represented in fig. 15. Resist- ance to Current. Every galvanic circuit offers a certain resistance to the passage of a current, both in the wires connecting the terminal plates, and in the fluids in the cells. From Ohm's law- there may be deduced many of the con- ditions upon which the strength of the galvanic current and the resistance offered to it depend. The primary force by which a galvanic cur- rent is set in motion is called the electromotive force, and this, upon the chemical theory, re- sides principally at the surface of the positive metal, and is generated by the chemical com- bination which takes place there. The quan- tity of current which is developed in a voltaic circuit depends upon the electromotive force and the resistance which it has to overcome in passing through the conductors, both solid and liquid, which are contained in the circuit. Ohm's law may be stated as follows : The strength of a galvanic current is equal to the electromotive force divided by the resistance, and is expressed by the equation C = ^, where C represents the strength of the current, E the electromotive force, and K the resistance. In an ordinary couple there are two resistances offered to the current: 1, that of the liquid conductor between the plates, called the in- ternal or essential resistance; and 2, that of- fered by the conductor connecting the two plates outside of the liquid. This conductor may consist of one or of several materials ; as a wire, or two or more wires having their ends placed in one or more liquids. The resistance offered by such external conductors is called the external or non-essential resistance. The strength of the current produced by increasing the size and number of the plates of a battery may be found by using the equation given above. Let the internal resistance be represented by, and the external resistance by t ; then in one couple C = j^-j. Let n represent the number of couples in a battery; then C = fflE = - which expression, if t is very small, as when the connection between the terminal plates is made by a short, thick copper wire, has very nearly the same value as that is, the strength of the current is not sensibly increased by increasing the number of couples when the circuit is closed by a good conductor. But if the external resistance is very great, as when the current passes through a long tele- graph wire, or through a liquid, its strength will be increased by increasing the number of couples. If the size of the plates is increased, then, according to the chemical theory, the electromotive force will be increased, and therefore the strength of the current, provided the conductors do not oppose too much resist- ance. According to the contact theory, in- creasing the size of the plates, and therefore the section of the liquid conductor between them, proportionately diminishes the internal resistance. If, therefore, the size of the plates is increased m times, = -^- = 7^7. If m the value of t is very small, the latter expres- sion has nearly the same value as f, or the