Carey Foster and Kelvin Bridges
   The Carey Foster bridge is often used for the measurement of very low resistances, although it can be used to find, for example, small differences between large resistances. The bridge is in two parts: a slide wire, connected by thick copper cables to a holder for standard resistances. The resistances hooked on the ends of the arms projecting up and down in the picture.

   G. Carey Foster was Professor of Physics at University College, London. Along with Alfred Porter of the same institution, he wrote "An Elementary Treatise on Electricity and Magnetism", founded on Joubert's "Traité Élémentaire D' Électricitié", a standard text in the years around the beginning of the 20th century which I have found useful.

   The Carey Foster Bridge in the picture is from Denison University, and was made by Leeds and Northrup of Philadelphia. In the 1903 L & N booklet describing the bridge there is an engraving of it with the name "Morris E. Leeds & Co., Phila.", the name of the original company. Its cost was $150.00.
 

The Kelvin Bridge is another form of low resistance bridge, invented by William Thomson, Lord Kelvin. The Students' Kelvin Bridge below was listed at $70.00 in the 1927 Leeds and Northrup catalogue. It could be used to measure resistances in the range from 0.001 ohm to 0.1 ohm with an accuracy to 0.2 percent. It is in the collection of the University of Vermont.

Below is a similar Kelvin Bridge in the Greenslade Collection.

The Kelvin bridge below was made by J.W. Queen of Philadelphia, and is at Westminster College in western Pennsylvania.

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