Telegraph relays amplify an electrical signal in a telegraph line. Telegraph messages travel as a series of electrical pulses through a wire from a transmitter to a receiver. The pulses fade in strength as they travel through the wire, limiting the distance a message can be sent. Relays remedy that problem by detecting a weak signal and automatically re-transmitting that signal down the line using a local power source.
Joseph Henry (1797-1878) made an international scientific reputation with his research into electromagnetism in the 1820s and 1830s. That reputation later played a role in his being named the first Secretary of the new Smithsonian Institution. This electromagnet is purported to be Henry’s first electromagnet. According to the donor’s father, inventor Amos Dolbear, “This electromagnet was given to Mr. Thomas Hall [of Boston], by Joseph Henry about 1870. Prof. Henry told Mr. Hall it was the first electromagnet he ever made. [The magnet , not its mounting.] Mr. Hall gave it to me in Oct. 1897. He understood that it was with this magnet the famous experiments in Albany in 1829 were made when Henry telegraphed thru the wire stretched round his lecture room."