572 



HISTORY OF SCIENCE. 



at any future period. By practice, however, the clerks recognize the 

 signals by the sounds made by the instrument, and they are frequently 

 able to write out the message by the ear alone. The sending in- 

 strument is simply a kind of spring lever, with which shorter or longer 

 contacts are made, so as to produce the dot and dash characters. 

 Figs. 288 and 289 show the operations of transmitting and receiving 

 a message by the Morse instruments. 



The scientific instrument constructed on the same principle as the 



FIG. 287. 



telegraphic needle is called the Galvanometer. One form of it is re- 

 presented in Fig. 290. A coil of fine copper wire, covered with silk, 

 is wound round a slender wooden frame, within which a magnetic 

 needle is free to move horizontally. This needle is usually attached 

 to another similar needle immediately above the coil. The two needles 

 are parallel, and their poles are in reverse directions. Two advan- 

 tages are gained by this : first, the directive force of the earth is re- 

 duced to merely the difference of its contrary actions on the poles ; 

 second, the action of the coil on the upper needle is concurrent with 

 the action on the needle within the coil. These astatic needles are sus- 

 pended by a single fibre of silk. An instrument of this kind indicates 

 the presence and the direction of the feeblest electric current. Thus, 

 if the terminals are connected by wires with two pieces of different 

 metals, the plunging of the metals into a tumbler of water will cause a 

 considerable deflection of the needle. The direction of this deflection 

 ascertained once for all for a known current, the instrument will always 

 indicate the direction of any current ; and indications of the relative 

 strengths of currents are afforded by observations of the angle of de- 

 flection, as shown on the graduated circle. The galvanometer is one 



