430 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 359. 



is sufSciently strong, it will place itself at risht angles with the wire, as is 

 represented in the tigure. 



If the current, however, had passed in the same direction below the needle, 

 instead of above it as in the first instance, the deflection ot the needle would 

 have taken place as before, but in an opposite direction, the pole S standing 

 where the pole N did previously, and N also in the place of S. 



In Uke manner, if the needle be placed by the side of the wire, a like effect 

 will be produced; on one side it dips down, and on the other it rises up; and 

 in whatever other position the needle may be 

 jiaced, it will always tend to set itself at right 



j-q, \. angles to the current. If the wire be bent in 



I N -^= ^ =^^ s [| the form of a rectangle, as is represented in Fig. 



359, so as to carry the current around tho 

 needle, above and below it in opposite direc- 

 tions, the opposite currents, instead of neu- 

 tralizing, will assist each other, and the needle 

 will move in accordance wdth the first direction of the current. 



If the wire, instead of making a single turn, is bent many times around the 

 needle, the magnetic force excited by the current of electricity traversing the 

 wire, will be greatly increased, the increase being, within cc:rtain limits, pro- 

 portional to the number of turns of tho wire. 



It is upon this principle that an instrument called the Gal- 

 G^v^ometer! vanometer, for measuring the quantity of an electric current, 

 is constructed. It consists of a rectangular coil of copper 

 wire, N B S, Fig. 360, containing about 20 

 convolutions, the separate coils being insulat- 

 ed by winding the wire with silk thread. A 

 magnetic needle, supported on a pivot, is 

 placed in the center of the coil, and a gradu- 

 ated circle is fixed below it to measure the 

 amount of the deflection; the two ends of the wire connect with two cups, 

 C and Z, which contain mercury, and into which the poles of tho battery 

 transmitting the current dip. 



In this form of the instrument 

 ^^c Xeedltr the transmitted current is obliged 

 to contend with the influence of 

 the earth's magnetism, which tends to hold tho 

 needle in its original position, and unless the 

 former is more powerful than the latter, the 

 needle is not moved. This difficulty has been 

 overcome by means of an arrangement called 

 the Astatic Needle. This consists essentially of 

 two needles fastened together, one above tho 

 other, but with their poles in opposite direc- 

 tions, as is represented in Fig. 361. In this 

 way the influence of the earth is almost entirely 

 emoved, and the force of the transmitted current is rendered more effective. 



Fig. 360. 



Fig. 36L 



