force.  It becomes a sort of magnetic popgun.  This is an experiment which has been twice discovered.  I found it first described by Count Du Moncel, in the pages of La Lumiere Electrique, under the name of the “pistolet electromagnetique;” and Mr. Shelford Bidwell invented it independently.  I am indebted to him for the use of this apparatus.  He gave an account of it to the Physical Society, in 1885, but the reporter missed it, I suppose, as there is no record in the society’s proceedings.

ELECTROMAGNETS FOR USE WITH ALTERNATING CURRENTS.

When you are designing electromagnets for use with alternating currents, it is necessary to make a change in one respect, namely, you must so laminate the iron that internal eddy currents shall not occur; indeed, for all rapid-acting electromagnetic apparatus it is a good rule that the iron must not be solid.  It is not usual with telegraphic instruments to laminate them by making up the core of bundles of iron plates or wires, but they are often made with tubular cores, that is to say, the cylindrical iron core is drilled with a hole down the middle, and the tube so formed is slit with a saw cut to prevent the circulation of currents in the substance of the tube.  Now when electromagnets are to be employed with rapidly alternating currents, such as are used for electric lighting, the frequency of the alternations being usually about 100 periods per second, slitting the cores is insufficient to guard against eddy currents; nothing short of completely laminating the cores is a satisfactory remedy.  I have here, thanks to the Brush Electric Engineering Company, an electromagnet of the special form that is used in the Brush arc lamp when required for the purpose of working in an alternating current circuit.  It has two bobbins that are screwed up against the top of an iron box at the head of the lamp.  The iron slab serves as a kind of yoke to carry the magnetism across the top.  There are no fixed cores In the bobbins, which are entered by the ends of a pair of yoked plungers.  Now in the ordinary Brush lamp for use with a steady current, the plungers are simply two round pieces of iron tapped into a common yoke; but for alternate current working this construction must not be used, and instead a U-shaped double plunger is used, made up of laminated iron, riveted together.  Of course it is no novelty to use a laminated core; that device, first used by Joule, and then by Cowper, has been repatented rather too often during the past fifty years to be considered as a recent invention.