. t 49 j 



VI. On some Electromagnetic Experiments with Open Cir- 

 cuits. By J. J. Thomson, B.A., Fellow of Trinity College, 

 Cambi ' 



T has been noticed incidentally by Helmholtz and others 

 that a coil acts in many cases like a condenser, and pos- 

 sesses appreciable electrostatic capacity. I am not aware, 

 however, of any experiments in which this property pro- 

 duced any. very marked effect. In the experiments I am about 

 to describe, where currents were induced in a secondary coil, 

 the secondary circuit was generally left open ; and it was 

 found that, under certain circumstances, the condenser-action 

 of the coil was great enough to produce local currents strong- 

 enough to powerfully magnetize a sewing-needle placed in- 

 side a spiral through which these local currents passed. The 

 following is a description of the coils used, which closely 

 resemble some used by Lord Rayleigh in some electromag- 

 netic experiments described in his paper " On an Electro- 

 magnetic Experiment," Phil. Mag. June 1870. 



Two wires, each about 300 yards long, were wound side 

 by side on a bobbin : one of these wires was used as the pri- 

 mary circuit ; the other, which was cut into three pieces 

 each about 100 yards long, formed three separate coils which 

 could be connected up in a great variety of ways, and which 

 were used as the secondary circuits. For convenience, I shall 

 denote the electrodes of the first of these secondary coils by 

 the letters A and B; the electrodes of the second by C and D; 

 and the electrodes of the third by E and F. As it will be con- 

 venient to have some notation to denote the way the secon- 

 dary coils are arranged, I shall signify the arrangement by 

 writing the letters denoting the electrodes in the order one 

 would meet with them if one were to travel continuously 

 along the wire of the coil. A small magnetizing-spiral, in 

 which an ordinary sewing-needle could be placed, was also 

 inserted in the secondary circuit. The position of this spiral 

 will be denoted by placing a — between the letters denoting 

 the electrodes between which it is placed : thus ABCD — EF 

 denotes that the electrode B was connected with the electrode 

 C, the electrode D to the electrode E, while the ends A ami 

 F were free ; the magnetizing-spiral was placed between D 

 and E. Again, AB— CDEF denotes that the electrode B 

 was joined to D, the electrode C to E, while the ends A and 

 B were free ; the magnetizing-spiral was placed between B 



and D. 



* Communicated by the Author. 



Phil. Mag. S. 5. Vol. 12. No. 72, July 1881, E 



