Electro-magnets play an extremely important part in the harnessing of electricity; in fact they are used in one form or another in almost every kind of electrical mechanism. In later chapters many of these uses will be described, and here we will mention only the use of electro-magnets for lifting purposes. In large engineering works powerful electro-magnets, suspended from some sort of travelling crane, are most useful for picking up and carrying about heavy masses of metal, such as large castings. No time is lost in attaching the casting to the crane; the magnet picks it up directly the current is switched on, and lets it go the instant the current is stopped. In any large steel works the amount of scrap material produced is astonishingly great, hundreds of tons of turnings and similar scrap accumulating in a very short time. A huge mound of turnings is awkward to deal with by ordinary manual labour, but a combination of electro-magnet and crane solves the difficulty completely, lifting and loading the scrap into carts or trucks at considerable speed, and without requiring much attention. Some time ago a disastrous fire occurred at an engineering works in the Midlands, the place being almost entirely burnt out. Amongst the débris was, of course, a large amount of metal, and as this was too valuable to be wasted, an electro-magnet was set to work on the wreckage. The larger pieces of metal were picked up in the ordinary way, and then the remaining rubbish was shovelled against the face of the magnet, which held on to the metal but dropped everything else, and in this way some tons of metal were recovered. The effect produced upon a magnetized needle by a current of electricity affords a simple means of detecting the existence of such a current. An ordinary pocket compass can be made to show the presence of a moderate current, but for the detection of extremely small currents a much more sensitive apparatus is employed. This is called a _galvanometer_, and in its simplest form it consists essentially of a delicately poised magnetic needle placed in the middle of a coil of several turns of wire. The current thus passes many times round the needle, and this has the effect of greatly increasing the deflection of the needle, and hence the sensitiveness of the instrument. Although such an arrangement is generally called a galvanometer, it is really a galvanoscope, for it does not measure the current but only shows its presence.