The Optical Magic Lantern Journal (November 1895)

182 The Optical Magic Lantern Journal and Photographic Enlarger. —————— _—oOoer ee—eee > rr mounted between two uprights, and above the disc is supported a needle capable of moving in a vertical direction. The apparatus can either be shewn by the first or second method, as described in the LanTERN Journnat, No. 77, page 155. The second method will dispense with the use of an erecting prism, as otherwise it would be necessary to show it upon the screen inverted. Fig. I. is a front elevation, and Fig. II. an end view of the piece of apparatus. would be advisable to make the magnet removable, as if kept in that position without a | keeper, it will become demagnetised. To do | this, it is advisable to secure the magnet in | position by means of small stops at each end. When arranged for projection upon the screen, it will appear as in Fig. III, By revolving the disc the freely-suspended needle will take up | its position according to the angle which the | magnet makes with the earth. The needle can be made from a piece of common watch-spring, and magnetised by any of the usual methods. A very simple arrangement, such as is shewn in Fig. IV., enables us to dispense with the ordinary horizontal needle. A piece of watchspring is magnetised, and placed upon a narrow strip of cork, and cemented to it if necessary. To each end in attached a cut out letter N 8 to their respective ends. A circular horizontal tank is placed upon the vertical attachment of the lantern and half filled with water. The mounted magnet is then floated upon the water and focussed upon the screen. By bringing a magnet or piece of magnetic substance close to the poles of the floating magnet—the usual experiments illustrating repulsion and attraction—the letters N and S will readily show upon the screen which end of the magnet is being repulsed or attracted according to the experiment in hand. When a piece of watchspring and cork are not available a common sewing needle can be magnetised, and, if gently dropped flat upon the surface of the water, it will be found that the strength of the surface film is sufficient to support the needle upon the water and give it freedom of motion. The needle must be perfectly dry, otherwise we shall meet. with failure in our experiment. Quite a number of pieces of apparatus have been devised from time to time to show by the aid of the lantern the strength of an electric current. Fig. V. is a sketch of one such as that used by Prof. J. A. Fleming to illustrate a It | lecture that he delivered to practical workers in electricity. The parts are very simple, and could easily be made by anyone capable of using his tools, and possessing a small knowledge of electricity. A hollow bobbin wound with a coil of silk-covered copper wire is mounted upon a wooden frame-work with a well-shaped opening cut out, and the sides formed of two lantern plate cover glasses. A short piece of soft iron with an aluminium pointer attached is supported on an axis. The axis lies just outside the opening of the hollow bobbin, and the soft iron needle lying inside the bobbin, as shown by the dotted = AM GY ui sf { o ARG rin NF d Fig. V. lines in the figure. The pointer when in its normal position rests obliquely in the coil. Upon passing a current of electricity through the coil of wire by connecting up by means of the binding screws to the source of electricity to be tested, the needle or projecting end in the bobbin tends to take up a direction in a line with the axis of the bobbin and the extent to which it is able to do this by overcoming the weight of the pointer, illustrates the strength of the current passing through the coil. Graduations made upon the glass sides to the well-opening show the extent of movement of the pointer. The piece of apparatus can be placed in the lantern as an ordinary old woodframed side. The mechanism of the pocket-watch was the subject of an interesting lecture delivered by H. FE. Duncan, December 7th, 1894, before the members of the Franklin Institute. Throughout the lecture the lantern was used to illustrate upon the screen the various pieces of mechanism that are contained in the watch. This is a subject that could well be more taken up in this country, and by the aid of the lantern be made to stimulate interest in the subject, both from a practical point of view both as regards the modern construction of the watch, and also of older types of watches exhibited in our museums. In cases where the lantern in some form or other could not be used to show the actual parts of the mechanism, either in motion or stationary, lantern slides could take its place.