In 1896 Professor Rutherford, now of Manchester, described some discoveries which he had made as to the magnetic effects of oscillations. A simple little contrivance which he had constructed was operated by the discharge of a coil half-a-mile away, at that time a great performance. This detector was simply an electro-magnet with a steel core instead of the usual soft iron core. The reason the latter is used in the ordinary magnet is that it loses its magnetism the moment the current ceases to pass through the coil with which it is surrounded, while a steel core retains its magnetism. For most purposes a steel core would render an electro-magnet useless, but in this case it was desired that the core should be permanently magnetised. So a current was first passed through the coil to magnetise the core, and then the coil was connected to a simple form of antenna while a swinging magnet was brought near so that the magnetic power of the core would be indicated and any change made apparent. The effect of the discharge half-a-mile away was to _de_magnetise the core slightly. This was shown by the movement of the swinging magnet, and so the first "magnetic detector" was found. But here, perhaps, I ought to explain the use of the antenna at the receiving station--its function at the sending end has already been made clear. The electro-magnetic waves, coming from the distant transmitter, strike the receiving antenna and in so doing _set up in it oscillations such as those which set them in motion_. For every oscillation in the sending antenna there will be another, similar in every respect except that it will be feebler, in the receiving antenna. And the oscillations are here led to the detector, of whatever form it may be, and in it they make their presence felt. In some few cases a Duddell thermo-galvanometer has been employed as the detector, in which the oscillating currents report themselves directly. In coherers the detector works by causing the oscillating currents to control a continuous current from a battery and it is the latter which actually gives the signal, but there are a number of extremely interesting means which have been invented to detect the oscillating currents by their heating effect.