112 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



60. The sensitive galvanometer (Kelvin type). From the equa- 

 tion of the tangent galvanometer, equation (36), it is evident that 

 a given current will produce the greatest deflection < of the 

 galvanometer needle when the number of turns of wire in the 

 coil is great, when the radius of the coil is small, and when the 

 directing field H' is weak. A galvanometer constructed so as 

 to meet these conditions and thus give a perceptible deflection 

 for a very small current is called a sensitive galvanometer. Such 

 a galvanometer is used chiefly for merely detecting the presence 

 of current in a circuit. The magnet of such a galvanometer is 

 usually suspended by means of a fiber of unspun silk or quartz, 

 and, in order that small deflections may be easily detected, a 

 mirror is usually attached to the suspended magnet so that the 

 angular movement of the suspended magnet may be observed by 

 means of a telescope and scale. 



Use of a governing magnet. In order to secure a weak 

 T directing field H 1 ', the earth's field may be partially neu- 

 tralized in the neighborhood of the suspended magnetic 

 needle by properly placing a large magnet in the neighbor- 

 hood of a galvanometer. This large magnet is called a 

 governing magnet. 



Use of an astatic system of magnetic needles. Two similar 

 magnetic needles NS and SN attached to a rod, as shown 

 in Fig. 77, constitute what is called an astatic system. Such a sys- 

 tem if suspended in the earth's magnetic field would point indif- 

 ferently in any direction if the two magnets were exactly alike 

 and exactly opposite in direction. If the two needles NS and 

 SN are nearly alike the earth's field will have but a very slight 

 directing action upon the system. Such a pair of magnetic 

 needles may be suspended with one of its magnets inside of a 

 galvanometer coil as shown in Fig. 78, or with its two magnets 

 surrounded by two properly connected coils as shown in Fig. 79, 

 and the result will be an extremely sensitive galvanometer. The 

 design shown in Fig. 79 is due to Lord Kelvin. A galvanometer 

 constructed after this design with very short magnetic needles, 



