Inductance by the Aid of a Vibration Galvanometer. 499 



Below the coil is a fastening of one or more silk threads c. 

 The range of frequency obtainable depends on the moment 

 of inertia of the moving pari", the tension, width of bifilars, 

 &c. In one specimen the ordinary range (from 50 to 100 -— 

 per sec.) can be obtained by simply tightening or loosening 

 the spring by the screw adjustment, while by placing an 

 adjustable bridge b (fig. 3) under the bifilars the range can 

 be extended to 700 or 800 ~ per sec. The readiness with 

 which the frequency can be adjusted appears to be one of the 

 advantages in the bifilar type. As the frequency is raised 

 the sensitivity decreases in the inverse ratio. With given 

 magnetic field, in normal use (i. e. with resonance) the 

 sensitivity only depends on the damping moment, which is 

 both mechanical and electrical. For example, if the moment 

 of inertia and the control torque be both increased in the 

 same proportion without altering the damping, then both the 

 frequency and the sensitivity remain unchanged. It is of 

 importance, therefore, to keep the damping small. For many 

 purposes,, at .frequencies of 20 to 200 ~ per second, sufficient 

 sensitivity can be obtained even when using a fairly large 

 mirror (1 cm. diameter), but for higher frequencies it is 

 advisable to reduce this size considerably. The control 

 torque is usually strong, the tension being of the order of 

 0*5 to 1 ko-in. 



5. Practical Working of Galvanometer and Huglies-Rayleigli 



Method. 



The best type of current to use is a nearly pure sine-curve 

 alternating current of very steady frequency (see Rosa and 

 Grover above), but an interrupted current can be used with 

 good accuracy. It is desirable to be able to set the frequency 

 of the current by gradual and fine adjustment for the exact 

 tuning, and for this purpose a wire interrupter like that of 

 Wien is effective. It is merely a monochord solidly supported 

 with fine adjustment of tension and maintained electrically 

 with a mercury break as tuning-forks are. When the 

 galvanometer is in resonance (which is known by the maximum 

 elongation of the spot of light with a given current), it does 

 not follow that it is responding to the fundamental frequency 

 given by the wire * ; it may be in resonance with one of the 

 harmonics. In order to determine the actual frequency to 

 which it is answering, a usual method is to test it by a 



* It is a curious fact that a Rubens Galvanometer with given control 

 sometimes has two points of retonance near one another, e. g. 40 and 43 

 -^- per second in one specimen. (See also Rosa and Grover, loc. cit.) 



