THE BOOK OF THE LANTERN. 



173 



FIG. 46. 



form the basis of the single needle electric telegraph, and 

 it is to demonstrate the powers of that telegraph that the 

 lantern galvanometer, which 

 is here figured, has been 

 devised. It will be noticed 

 that on its right-hand side it 

 is furnished with two ter- 

 minals. These are connected 

 with the ends of the coil 

 wire, and provide a means of 

 readily joining the instrument 

 up to the battery, placed in 



any position outside the lantern ; but in practice it will 

 be found advisable to also place in connexion with this 

 slide and its battery a little piece of apparatus which may 

 be called a "current reverser," which can easily be made 

 at home. It is shown in fig. 46. It consists of two 

 little treadles formed out of brass. This brass, it may 

 be mentioned, should be of the hard-rolled kind, such as 

 is used for springs. 



Each of these little treadles is fastened down to a maho- 

 gany board, which forms the base of the instrument, and 

 each one is in connexion with a terminal screw, which is 

 indicated in the cut by a round dot. Across the other end 

 of the treadles is a raised bar of brass, against which they 

 spring up and touch when in their normal condition ; but 

 when either of them is pressed down, it touches a piece of 

 brass wire which is let into the top of the mahogany board 

 immediately underneath. 



This wire, as well as the piece of brass just mentioned, 



