Page:The American Cyclopædia (1879) Volume VII.djvu/606

 594 GALVANISM strength of the current then increases very nearly in proportion to the increase in the size of the plates ; but if the external resistance is great, the strength of the current will not be increased in proportion to the increase in the size of the plates. Hence, in magnetizing soft iron by passing a current around it through a coil of stout wire, it will be of advantage to use a small number Of large couples; but in FIG. 16. Coupling for Intensity. passing a current through a long wire or an electrolyte, or any poor conductor, a large number of couples is to be preferred. The coupling to overcome external resistance is represented in fig. 16, which is the arrange- ment adopted in telegraph batteries and in galvanoplastic operations, and is called coupling in series. Coupling for quantity, or, as it is sometimes called, coupling in multiple arc, is represented in fig. 17, where plates of the same metal are grouped together. It has the same effect as the employment of one pair of plates having an equal area of surface ; increasing the sectional area of the internal or fluid conduc- tor, and correspondingly diminishing the inter- nal resistance ; also increasing the quantity of current through the external conductors when they are of sufficient capacity, or when they offer enough resistance, of generating an equiva- lent quantity of heat. The tension of galvanic is far less than that of frictional electricity, but by greatly multiplying the number of pairs in a battery the tension of frictional electricity FIG. 17. Coupling for Quantity. may be approached. Thus, a battery con- structed by Mr. Gassiot of 3,520 zinc and cop- per pairs, having electrodes -^ of an inch apart, gave a series of sparks across the interval which lasted for several months. It is usual to say that the quantity of frictional electricity is small and its tension great, while the quantity of a galvanic pair is great in proportion to its ten- sion. An experiment of Faraday's showed that two wires, one of zinc and one of plati- num, each -fa of an inch in diameter, im- mersed in acidulated water -/$ of a second, had a greater effect on a magnetic needle than that of 23 turns of the large electrical machine of the royal institution. The physiologi- cal effects of galvanic elec- tricity vary with intensity and quantity ; they are treat- ed in the article MEDICAL ELECTRICITY. The astatic gal- vanometer cannot be used to measure currents of much strength, on account of its too great delicacy. For this purpose the tangent galvan- ometer and the sine galvan- ometer are employed. The tangent galvanometer con- sists of a vertical circle made of a band of copper the two ends of which are connected with the poles of a battery. In the centre of this vertical circle a small magnetic needle is placed, in length about T V of the diameter of the circle. "When the needle is no longer than this, the tangent of the angle of deflection will be proportional to the strength of the current. In using the instrument the plane of the verti- FIG. 18. Tangent Galvanometer. FTG. 19. Sine Galvanometer. cal circle is placed in the plane of the magnet- ic meridian. The sine galvanometer, invented by Pouillet, is represented in fig. 19. A longer magnetic needle may be employed in this in-