140 THE POPULAR SCIENCE MONTHLY. 



in the length of the rod changes the pressure upon the carbon, and 

 alters the resistance of the circuit. This causes a deflection of the g-al- 

 vanometer-needle — a movement in one direction denoting expansion of 

 A, while an opposite motion signifies contraction. To avoid any deflec- 

 tion which might arise from change in strength of battery, the tasimeter 

 is inserted in an arm of the Wheatstone's bridge. 



Fig. 33. 



In order to ascertain the exact amount of expansion in decimals of 

 an inch, the screw 8, seen in front of the dial, is turned until the deflec- 

 tion previously caused by the change of temperature is reproduced. 

 The screw works a second screw, causing the rod to ascend or descend, 

 and the exact distance through which the rod moves is indicated by the 

 needle, N^ on the dial. The instrument can also be advantageously 

 used to measure changes in the humidity of the atmosphere. 



In this case the strip of vulcanite is replaced by one of gelatine, 

 which changes its volume by absorbing moisture. 



The Aeephone. — The arephone, an invention of Mr. Edison's, for 

 amplifying sound, has already attracted considerable attention, though 

 as yet it has not been perfected. 



Its object is to increase the loudness of spol^en words, without im- 

 pairing the distinctness of the articulation. 



The working of the instrument is as follows : The magnified sound 

 proceeds from a large diajDhragm, which is vibrated by steam or con- 

 densed air. The source of power is controlled by the motion of a second 

 diaphragm, vibrating under the influence of the sound to be magnified. 



There are three distinct parts to the instrument : 



A source of power — compressed air. 



An instrument to control the power. 



A diaphragm vibrating under the influence of the power. 



The first of these is usually compressed air, supplied from a tank. 

 It is necessary that it should be of constant pressure. 



The second, shown in section at Fig. 34, consists of a diaphragm and 

 mouth-piece, like those used in the telephone. A hollow cylinder is 

 attached by a rod to the centre of the diaphragm. The cylinder, and 

 its chamber, E^ will therefore vibrate with the diaphragm. 



