THE ELECTKIC BELL. 729 



has an aperture in the middle, through which a button, 

 d, usually of porcelain > passes. The button is pressed 

 underneath by the spring &, but is kept from being 

 forced out by a small ledge around it, which is pressed 

 by the spring against the cover. When the button is 

 pressed down, the spring b com.es in contact with the 

 metal piece a, and the circuit is completed, because a 

 and b are respectively connected with the wires leading 

 to the poles of the battery, the ends of these wires 

 being clamped by the heads of the screws which serve 

 for fastening a and b to the wooden disc. The key is 

 of course fixed at the place from which it is desired 

 to ring the bell ; the battery may be erected near the 

 bell if convenient, or near the key, or anywhere be- 

 tween them. One of the binding screws of the bell is 

 connected with one pole of the battery, the other bind- 

 ing screw with one of the metal strips of the key, and 

 the second metal strip of the key is connected with the 

 other pole of the battery. The bell then rings and will 

 continue to ring as long as the button is kept pressed 

 down. 



Meidinger's elements are the best for electric bells, as they need 

 only be renewed about once a year. One cell will give a sufficiently 

 strong current for a single bell, if the circuit is not too long. 

 Copper wire should always be used for the connections ; iron wire 

 soon becomes rusty, and then breaks very easily unless it is very 

 thick ; but in that case it is inconveniently stiff and is not easily bent 

 so as to fit the angles of walls, etc. Copper is also by far the better 

 conductor. Copper wire covered with cotton thread of various 

 colours, so as to suit the colour of the wall along which the wire 

 is fixed, is often sold for electric bells, but is not to be recom- 

 mended ; the insulation of the covering is soon rendered imperfect by 

 the wearing away of portions of the thread, in consequence of which 

 the moisture of damp walls finds its way between the wire and the 

 covering. Common uncovered copper bell wire, l mm thick, answers 



