400 Prof. J. A. Fleming. Magnetic Detector for [Feb. 11,. 



circuit of the magnetising coils placed round the iron bundles, and 

 thus by applying a magnetising current to magnetise them during a 

 portion of one period of rotation of the disc, and leave them mag- 

 netised in virtue of magnetic retentivity during the remaining portion. 

 The function of discs 2 and 3 is to short-circuit the terminals of the 

 secondary coil of the bobbin during the time that the magnetising 

 current is being applied by disc No. 1. A sensitive movable coil 

 galvanometer is employed in connection with the secondary coil, one 

 terminal of the galvanometer being permanently connected to ona 

 terminal of the secondary coil, and the other terminal connected 

 through the intermittent contact made by the disc No. 4. This disc 

 No. 4 is so set that during the time that the secondary coil is short- 

 circuited, and whilst the battery current is being applied to magnetise 

 the iron wire bundles, the galvanometer circuit is interrupted by the 

 contact on disc No. 4. 



The operations which go on during one complete revolution of the 

 discs are as follows : — First the magnetising current of a battery of 

 secondary cells is applied to magnetise the iron bundles, and during 

 the time this magnetising current is being applied, the terminals of 

 the fine wire secondary coil are short-circuited and the galvanometer 

 is disconnected. Shortly after the magnetising current is interrupted 

 the secondary bobbin is unshort-circuited, and an instant afterwards 

 the galvanometer circuit is completed and remains completed during 

 the remainder of one revolution. Hence, during a large part of one 

 revolution, the iron wire bundles are left magnetised, but the mag- 

 netising current is stopped and the galvanometer is connected to the 

 secondary coil. If during this period an electrical oscillation is passed 

 through the demagnetising coils, an electromotive force is induced in 

 the secondary bobbin by the demagnetisation of the iron and causes 

 a deflection of the galvanometer coil. Since the interruptor discs 

 are rotating very rapidly, if the electrical oscillation continues, these 

 intermittent electromotive impulses produce the effect of a continuous 

 current in the galvanometer circuit, resulting in a steady deflection, 

 which is j)roportional to the demagnetising force being applied to the 

 iron, other things remaining equal. If the oscillation lasts only a very 

 short time, the galvanometer will make a small deflection ; but if the 

 oscillation lasts for a longer time, then the galvanometer deflection 

 is larger, and tends to become stead} 7 . 



By means of such an arrangement it is possible to verify the law 

 according to which variation falls off with distance. The instrument 

 can be employed also as a telegraphic receiving instrument, but its 

 chief use will be for comparing together the wave-making power of 

 different radiators. For this purpose the oscillation coils must be con- 

 nected to two long connecting wires, or one end may be connected to 

 the earth and the other to a vertical aerial. This detector serves, for 



