Thus, if a needle be inclosed in a wire not touching it at any point, and a current of electricity pass through the wire, the needle will be made to move in accordance with the direction of the current.
What other important discovery was made about the same time?
It was found that if a piece of soft iron, not possessing magnetic power sufficient to elevate a grain weight, be placed within a coil of copper wire through which a galvanic current is passing, it will become, through the influence of the current, a powerful magnet; and will, so long as the current flows, sustain weights amounting to many hundreds of pounds.
Is the magnetic power of the bar found to be wholly dependent on the existence of the current?
It is; the moment the current stops, the weights fall away from the bar in obedience to the law of gravity.
How great weights have been lifted by magnets formed in this manner?
An electro-magnet constructed by Prof. Henry was capable of elevating and sustaining about a ton weight.
Upon what principle does the construction of the Morse magnetic telegraph depend?
Upon the principle that a current of electricity circulating about a bar of soft iron is capable of rendering it a magnet.
Why is it necessary, in conveying the telegraph wires, to support them upon glass or earthen cylinders?