300 



SCIENCE OP COMMON THINGS. 



Excitation of Magnetism. Morse's Magnetic Telegraph. 



Fig. 94. 



Fig. 95. 



193*7 Is Hie magnetic power of the bar found to be wholly dependent on 

 (he existence of the current f 



It is / the moment the current stops, the weights 

 fall away from the bar in obedience to the law of 

 gravity. 



1933 How great weights have been lifted by magnets formed in this 

 manner 1 



An electro-magnet constructed by Prof. Henry was 

 capable of elevating and sustaining about a ton weight. 



1939 Upon what principle does the construction of the Morse magnetic 

 telegraph depend ?. 



Upon the principle that a current of electricity circu- 

 lating about a bar of soft iron is capable of rendering it 

 a magnet. 



The arrangement by which this principle is made available in the con- 

 struction and operation of the Morse magnetic telegraph will be under- 

 stood by reference to the accompanying diagram (Fig. 96), which repre- 

 sents the construction and arrangement of this form of telegraph. F and 

 E are pieces of soft iron surrounded by coils of wire, which are connected 

 at o and b with wires proceeding from a galvanic battery. When a cur- 

 rent is transmitted from a battery located one, two, or three hundred 

 miles, as the case may be, it passes along the wires and into the coils sur- 

 rounding the pieces of soft iron F and E, thereby converting them into 

 magnets. Above these pieces of soft iron is a metallic bar or lever, A, 

 supported on its centre, and having at one end the arm D, and at the 

 other a small steel point, o. A ribbon of paper, p h, rolled on the cylinder 

 B, is drawn slowly and steadily off by a train of clock-work, K, moved by 

 the action of the weight P on the cord C. This clock-work gives motion 

 to two metal rollers, G and H, between which the ribbon of paper passes, 

 and which, turning in opposite directions, draw the paper from the cylin- 

 der B. The roller H has a groove around its circumference (not repre- 

 sented in the engraving) above which the paper passes. The steel point, 

 9, of the lever, A, is also directly opposite this groove. The spring r pre- 



