English: An experimental mercury arc vacuum tube developed as an amplifying repeater for telephone lines around 1912 by Harold D. Arnold at Western Electric the manufacturing arm of the Bell Telephone Company. This is one of several crude technologies which telephone companies used to amplify telephone signals before the adoption of the Audion (triode) vacuum tube in 1913, and is notable chiefly as an amplifier tube that predated the vacuum tube. Arnold's tube amplified adequately, but was so difficult to adjust that it was only used briefly in one long distance line. The tube functioned by negative resistance cancelling the positive resistance of the telephone line, so it had to be adjusted carefully to the individual resistance of each line, or it would oscillate, producing shrieks and whistles on the line.