214 



ON MAGNETISM. 



is exceedingly valuable, because by it we can make and 

 unmake a magnet at a great distance, even several 

 hundred miles, and in any locality, and even in a 

 moving frame. 



A convenient and powerful form is that of the 

 horse-shoe magnet, the wires being arranged as in 

 Figure 72. A piece of iron must be provided, to be 



Fig. 72. 



pulled by the two poles of the magnet. It is in this 

 form that galvanism is commonly employed for the 

 telegraphs in which permanent impressions are made 

 on paper at the distant station. 



90. Spontaneous terrestrial galvanic currents : in- 

 vestigation of the magnetic effects due to them, and 

 comparison of these magnetic effects with the magnetic 

 disturbances recorded by the self-registering magneto- 

 meters. 



In the ordinary system of wire-telegraphs, each 

 wire, when not used in the actual work of transmitting 

 galvanic currents, is detached from all galvanic bat- 

 teries, and is connected at both ends with the earth. 



