Bibliography:

1. Nineteenth-century Scientific instruments. London Berkely:Sotheby Publications University of California Press,1983
2. A manual of experiments in physics: Laboratory instructions for College Classes.Joseph Sweetman Ames, William Julian Albert Bliss.
3. Maxwell, James. Treatise on electricity and magnetism. Cambridge: Cambridge University Press, 2010.
4. Introduction to Physical Science by Alfred.D.Bage,Forgotten books.

Webliography :

1. http://en.wikipedia.org/wiki/Galvanometer#Tangent_galvanometer  
2. www.mccc.edu/~huangj/documents/gal.pdf