498 Mr. A. Campbell on the Measurement of Mutual 



due to Max Wien *, in one of whose papers ( Wied. Ann. xliv. 

 p. 689, 1891) will be found a very complete discussion of a 

 number of his methods of measuring inductance and capacity. 



In all his earlier experiments the tuned instrument was an 

 " Optical Telephone/' in which the motion of the diaphragm 

 was magnified by the use of a small mirror with light spot 

 and scale. The sensitivity of this was 3 x 10 -7 amp. per mm. 

 at 1 m. 



In 1896 Rubens's Vibration Galvanometer appeared t; it 

 consists of a series of very small magnets or soft-iron needles 

 fastened to the middle of a tightly strung torsion wire and in 

 a field due to two strong permanent magnets, round whose 

 pole-pieces are coils carrying the alternating current to be 

 measured. The tuning is effected by altering the effective 

 length of the clamped torsion wire and by adjustment of the 

 magnets, and the sensitivity obtained is said to be four times 

 greater than that of the optical telephone. 



Some years later % M. Wien brought out a more sensitive 

 form, in which the small magnet system is between the poles 

 of a small electromagnet magnetized entirely by the current 

 to be measured. 



4. Moving Coil Vibration Galvanometer. 



After using an instrument of the Rubens type for some 

 time, I designed another of moving coil type, which I have 

 found more convenient. 



It consists of an electromagnet (or permanent magnet) 

 with a rather narrow air gap in which is suspended a very 

 light and small coil with bifilar control, which can be regu- 

 lated by altering the tension by means of an adjustable spring 

 or weight (as in some oscillographs). 



Fig. 3. 



65-; 



Spring 





BlF/LARS 



Si lh Threads 



In fig. 3 is shown one arrangement of the suspended 

 system which I have used (shown horizontally for convenience 

 of printing). 



* Max Wien, Wied. Ann. xlii. p. 593 (1891); xliv. p. 681 (1891) ; 

 xliv. p. 689 (1891) ; lvii. p. 249 (1896) ; lviii. p. 353 (1898). See also 

 E. Orlich, Elektrotechn. Zeitschr. vol. xxvi. (1903). 



+ Rubens, Wied. Amu lvi. p. 27 (1896). 



\ M. Wien, Ann. der Plnjsik, iv, p. 425 (1901). 



