52 Mr. J. J. Thomson on some Electromagnetic 



secondary circuit ; then, when the remaining coils were ar- 

 ranged according to the scheme AB — DC, a needle placed 

 in the magnetizing-spiral vibrated only 7 times a minute ; 

 whilst if the coils were arranged according to the scheme 

 BA — DC, a needle vibrated 36 times per minute. This dif- 

 ference can be explained in the same way as the correspond- 

 ing one in the case of three coils. 



A very marked difference between the behaviour of closed 

 and open circuits is shown if, instead of breaking the primary 

 circuit by a key in the ordinary way, we connect the points 

 where the circuit would be broken with the opposite plates of 

 a condenser of large capacity, so that at the break, instead of 

 a spark passing, we have the electricity rushing into and 

 charging the condenser. With a closed secondary this has 

 the effect of slightly increasing the intensity of magnetization 

 of a needle placed in the magnetizing-spiral (see Lord Bay- 

 leigh's paper " On an Electromagnetic Experiment," Phil. 

 Mag. June 1870); but with an open secondary circuit it has 

 in general a most marked effect in the opposite direction, in 

 some cases, indeed, appearing to produce hardly any magneti- 

 zation at all. This is shown in the following statement of the 

 result of some experiments : — 





Number of vibrations per 



Number of vibrations 



Scheme of coils. 



minute with condenser in 



"without condenser in 





primary circuit. 



the primary. 



ABFE— DC 



5 



28 



BA— DC 



6 



36 



BA— DC 



4 



33 



In the last experiments the coil EF did not form a part of 

 the secondary circuit. 



The effect is thus very marked. An exception to this, 

 however, occurs when, instead of using the electrostatic capa- 

 city of the coil itself to produce the magnetization of the 

 needle, we arrange the coil so that its electrostatic capacity is 

 small, but increase it artificially by connecting the free ends 

 of the coil with the opposite plates of a condenser. Under 

 these circumstances the introduction of a condenser into the 

 primary does not produce nearly so much difference in the 

 magnetization of the needle. This is shown by the following 

 experiments, in which the secondary coil was arranged accord- 

 ing to the scheme ABCDEF, which gives the least electro- 

 static capacity to the coil ; but the ends A and F were con- 

 nected with the opposite plates of a condenser whose capacity 

 was about one third of a microfarad. The magnetizing-spiral 

 was placed between A and one of the plates of the condenser. 



