172 THE BOOK OF THE LANTERN. 



a central pin, which is shown by the screw slit in the cut. 

 A pane of glass of a semicircular shape forms a back- 

 ground for the needle ; and this glass can either be left 

 plain, as in the illustration, or it can have drawn upon it a 

 scale. The slide figured is without the arrangement just 

 mentioned, and it is one that I have employed for a special 

 purpose. I have used it as a means of demonstrating the 

 action of the needle telegraphic instrument ; and it will 

 be noticed that two little buttons are fastened to the 

 glass in order to prevent the needle making too wide an 

 excursion. 



It may be mentioned here for the benefit of those who 

 are unused to electrical instruments, that a galvanometer 

 furnishes the means of detecting the existence of an elec- 

 tric current. In its higher forms it is so sensitive that 

 a current, generated by touching two dissimilar metals 

 with the fingers and excited by the natural warmth of the 

 hand, can, by a galvanometer, be made evident to the eye. 

 The most simple form of galvanometer can be readily 

 made from one of those little charm compasses which are 

 sold at the opticians' for about Is. each. Take such a 

 compass, and bind it across with several layers of fine silk- 

 covered copper wire. Place it in such a position that the 

 wire coil lies parallel with the needle, which will, of course, 

 be north and south ; now join the ends of the wire to 

 any form of electric battery, and the needle will imme- 

 diately swing round and take an east and west direction. 

 By changing the position of the wires with regard to the 

 poles of the battery, it will be noticed that the needle is 

 deflected in the opposite direction. These phenomena 



