Infos from the Web
Cable Key by Nalder Bros. of London
Double tapping key by Nalder Brothers
Collection: SCM - Telecommunications
Object Name: double tapping
Maker: Nalder Brothers and Company
Description: Double tapping key no. 10446, by Nalder Brothers and Co., Westminster
[Trade catalogs from Nalder Bros. & Thompson, Ltd.]
Established 1884
Company Name: Nalder Bros. & Thompson, Ltd.
Related companies: Nalder Bros. & Co. (Westminster, England)
Place: London, United Kingdom
Date: 1900s
Nationality: British
Brief Bio: active 1880s(?) - 1909(10), electrical measurement instrument maker
Notes: Traded at Westminister Engineering Works, 132 Horseferry Rd., Westminister (1890),
16 Red Lion St.(1892-3), 7 Carteret St., Queen Anne Gate (1910) & works, Dalston Lane, Hackney (1899-1909/10), all London
The rigorous environment imposed by extremely long distances required new technology.
There was not enough current in the early cables to pull the electromagnets of a telegraph sounder or relay.
To copy the weak signals a mirror galvonometer was used on the receiving end.
Galvonometers are very sensitive to weak currents and hence the movement of a galvo could be detected.
The receiving operator watched the galvo swing left and right.
On the sending end, keys with two pole were used to make the dots and dashes. The keys had two levers.
The left lever was used to send dots. Pressing it drove the cable voltage positive. The right lever was used to send dashes and drove the cable negative. Since the cable was thousands of miles long, it acted as a huge capacitor and the full plus-to-minus voltage was necessary to help it discharge.
The cable messages were usually recorded on a moving paper strip and it took skill to visually tell the difference between dots & dashes.
The rigorous environment imposed by extremely long distances required new technology. 
