Notes and Reflections on Books and Media
by Hannah Leitheiser
The Story of the Atlantic Telegraph
This is the state-of-the-art ammeter from the 1850's. Coil attracts magnets attached to a mirror. Deflection is noticed in the deflection of the lamplight.
This was the device used to receive signals across the Atlantic telegraph wire. To send, the wire was connected to a battery or to ground. Of course, it took awhile for the DC signal to overcome the impedance of the wire and show up on the other end. Seems to me short pulses might have worked better, or better yet, a modulated carrier wave, but I don't think that sort of thing wouldn't happen until spark-gap transmitters and the era of wireless telegraphy.