278 Dr. R T. Trouton on the Theory of Use of a 



With such an apparatus it is easy to arrange for showing 

 the gain in sound brought about by permanently magnetizing 

 the core of the electromagnet. For this purpose an alter- 

 nating current is employed of insufficient intensity to affect 

 the diaphragm much ; one then brings up the permanent 

 magnet to the position shown in the diagram, on which the 

 sound is found to be immensely improved. A similar result 

 of course can be obtained by a permanent current sent round 

 the additional coil. 



To distinguish between the parts of this improved effect 

 due respectively to the two causes mentioned above, one can 

 either connect up the additional coil wound on the electro- 

 magnet with an electro-dynamometer or with a telephone. 

 If the improved action on bringing up the steel magnet were 

 entirely due to the second cause (namely the fact that then 

 the same current is able to produce more lines of force in the 

 iron than before), the deflexion of the dynamometer or the 

 sound in the telephone would increase in like proportion with 

 the loudness of the tambourine. The experiment proves that 

 though there is undoubtedly a slight increase in the number 

 of lines of force, shown by a slight increase in the deflexion of 

 the dynamometer, the chief improvement is to be referred to 

 the other cause. 



A similar experiment was made with the additional coil 

 wound on the armature instead of on the core. This was 

 effected by taking a rather thicker piece of iron than that 

 which had been previously employed for the arming of the 

 diaphragm of the tambourine. On connecting up a telephone 

 with the coil, it could be arranged so as to have the sound 

 from the telephone and from the tambourine, with the steel 

 magnet removed, of equal intensity ; then, on bringing up 

 the magnet as before, the sound of the tambourine is found 

 to be far and away louder than the telephone. 



Lest it might be thought that the increased effect could be 

 due to the diaphragm coining nearer to the electromagnet in 

 consequence of the attraction arising from the permanent 

 magnet, it may be mentioned that similar effects are pro- 

 duced on placing the magnet on the opposite side to that as 

 shown in the diagram. In this position the diaphragm lies 

 between the magnet and the electromagnet, and consequently 

 tends to be drawn further away from the electromagnet. 



A somewhat curious effect is to be observed with this 

 apparatus on employing an intermittent current instead of an 

 alternating one. With an alternating current a continuous 

 increase in sound is of course produced indifferently by either 

 pole of the permanent magnet as it is brought up from a dis- 



