Drawing of a <a href="https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin" class="extiw" title="en:William Thomson, 1st Baron Kelvin">Thomson</a>  <a href="https://en.wikipedia.org/wiki/Galvanometer" class="extiw" title="en:Galvanometer">reflecting galvanometer</a> from around 1880, showing use.  The vertical tube contains a fine silk fiber, which suspends a 1/2 in. mirror with small magnets on its back, seen in window, in the center of a coil of wire.  A light beam reflected from the mirror lands on a scale and serves as a pointer. <a rel="nofollow" class="external text" href="http://www.chss.montclair.edu/~pererat/mpersci.htm">Tom Perera's Scientific instrument collection</a> says this was a Model 502, made by Elliot Bros., London. It has interchangeable coils of from 150 to 5000 ohms resistance. The left drawing is from Schneider, the right from Thompson. Alterations: removed captions, rotated Schneider image so it was vertical, scaled Thompson image by 0.60, combined images.
Norman Hugh Schneider and Silvanus P. Thompson
1913
Downloaded from <a rel="nofollow" class="external text" href="https://books.google.com/books?id=ZZ83AAAAMAAJ&amp;pg=PA13&amp;sig=v31U_QMaInWIfs6Q2g7aSCtkHrQ">Norman H. Schneider (1913) <i>Electrical Instruments and Testing</i>, Spon &amp; Chamberlain, New York, p.13 fig.8</a> and <a rel="nofollow" class="external text" href="https://books.google.com/books?id=2-sDAAAAQAAJ&amp;pg=PA169">Thompson, Silvanus P. (1881) <i>Elementary Lessons in Electricity and Magnetism</i>, Macmillan, London, p.169, fig.91</a> on Google Books