Masterpieces of Science 



needle, which will instantly sway about as if 

 blown hither and thither by a sharp draught of 

 air. This action takes place if a slate, a pane of 

 glass, or a shingle is interposed between the 

 needle and its perturber. There is no known 

 insulator for magnetism, and an induction of this 

 kind exerts itself perceptibly for many yards 

 when large masses of iron are polarised, so that 

 the derangement of compasses at sea from moving 

 iron objects aboard ship, or from ferric ores 

 underlying a sea-coast, is a constant peril to the 

 mariner. 



Electrical conductors behave much like mag- 

 netic masses. A current conveyed by a .con- 

 ductor induces a counter-current in all surround- 

 ing bodies, and in a degree proportioned to their 

 conductive power. This effect is, of course, 

 greatest upon the bodies nearest at hand, and we 

 have already remarked its serious retarding 

 effect in ocean telegraphy. When the original 

 current is of high intensity, it can induce a per- 

 ceptible current in another wire at a distance of 

 several miles. In 1842 Henry remarked that 

 electric waves had this quality, but in that early 

 day of electrical interpretation the full signifi- 

 cance of the fact eluded him. In the top room 

 of his house he produced a spark an inch long, 

 which induced currents in wires stretched in 

 his cellar, through two thick floors and two rooms 

 which came between. Induction of this sort 

 causes the annoyance, familiar in single tele- 

 phonic circuits, of being obliged to overhear 

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