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Rh, and back to the battery again by this other wire. Such a current we may call a continuous current. Now let us see what effect a current of this kind has on a nerve connected with a muscle. We connect the wires of the battery with two platinum wires,

over which the nerve has been stretched, and the muscle, as before, is connected with the muscle telegraph bell. To enable us to stop the passage of the current, or to send it on at pleasure, we place in the circuit what is called a key. It consists of a rectangular wooden