Chemistry and Physics. 97 



liquid with a clearly defined meniscus, must be regarded as an 

 achievement of a greatly higher order. — Nature, lviii, 55-57, 

 May 18th, 1898 ; Phil Mag., V, xlv, 543, June, 1898. g. f. b. 



8. Some New Methods for the Measurement of Self- Inductance, 

 Mutual Inductance and Capacity ; by H. A. Rowland and T. D. 

 Penniman. (From the Johns Hopkins University Circulars, No. 

 135, June, 1898.) — In the American Journal of Science, December, 

 1897, and in Phil. Mag., January, 1898, one of us published an ac- 

 count of a large number of new methods for the measurement and 

 comparison of self-inductance, mutual inductance and capacity. 

 Several of these methods have been tested in the Physical Labora- 

 tory of the University with great success, notably the methods 

 for the comparison of the above quantities. 



The methods involve the use of the electrodynamometer with 

 an alternating current, and depend on one general principle — that 

 is, that the deflection of the hanging coil of an electrodynamom- 

 eter, if the hanging coil and the fixed coils of the electrodyna- 

 mometer are originally at right angles, is proportional to the 

 product of the currents in the hanging coil and the fixed coils, 

 together with the cosine of the phase difference of the two cur- 

 rents. The dependence of the deflection on the cosine of the 

 phase difference of the currents divides the methods into two 

 general classes — methods in which there is a deflection, i. e., the 

 cosine of the phase difference has a value; and methods of zero 

 deflection, i. e., the cosine of the phase difference is equal to zero. 

 The first class of methods gives the self-inductance or capacity in 

 terms of resistance and b, i. e., 2tt (the number of complete alter- 

 nations per second of the current). These methods were very 

 easy of application ; the chief difficulty was the variation of &, 

 as the current was generated in the power house of the Uni- 

 versity, where the engine was subject to great change of load. 

 The presence of electric absorption also interfered with the accu- 

 rate determination of the capacity of condensers. 



The following method is an example of this class, and of the 

 accuracy with which self-inductance can be determined even 

 under these conditions. In this method the fixed coils of the 

 electrodynamometer are in the main line or circuit, and the hang- 

 ing coil is shunted ofl the main circuit around a small resistance, 

 r. In the hanging-coil circuit is placed a non-inductive resistance, 

 R, with a self-inductance, X, under which condition the hanging 

 coil will have a certain deflection when the current is flowing. 

 They, R and X, are now removed, and a non-inductive resistance, 

 R\ is substituted, which is adjusted to give the same deflection 

 as when R and L were in circuit. Under these conditions, 



b*L* = (R 1 -R)(R + r). 



By this method and similar methods given in the article cited, 

 values of self-inductance were determined that agreed among 

 themselves to within about 1 part in 100, i. e., the determinations 

 were made with about the same degree of accuracy as b could be 



ku. Jour. Sci.— Fourth Series, Yol. YJ, No. 31. — July, 1898. 

 7 



