The Callendar and Griffiths bridge is contained in a 50cm-long, 27cm-wide and 17cm-tall wooden box with a 7cm-tall lockable and openable lid. The body of the bridge itself is 46cm long, 23cm wide and 10cm tall and it is fixed on a 49cm-long and 26cm-wide wooden board. Unlike more contemporary and newer versions of Callendar and Griffiths bridges, the external box and the bridge itself are not seamlessly fitted between each other. Instead, there is a gap between the bridge and the inner sides of the box. There was a brass hinge in the lower-left corner of the box in the aforementioned gap, allowing the bridge to be lifted out of the box while secured by the hinge. The panel of the bridge is consisted of three parts. Two parallel brass rods outside and two iron alloy strings inside together form a rail on the lower side of the panel, allowing a jockey sliding from left to right. In the middle there are eight brass terminals with plastic-capped switches. Different readings are marked on the brass terminal seats as well on the corresponding plastic caps: 10, 20, 40, 80, 160, 320, 640, 1280. On the upper side there are eight brass coils.
Accession Number: 2010.ph.302
Callendar & Griffiths Bridge, Callander-Griffiths Bridge, Callender and Griffiths Bridge
Copper alloy, Wood, Slate, Silver, Platinum, Mica
On a metal bar within the case: “THE CAMBRIDGE SCIENTIFIC INSTRUMENT CO. LTD. CAMBRIDGE ENGLAND”
“197” (This marking appears to have been added post-manufacture)
“CALLENDAR & GRIFFITHS BRIDGE”
There are numeral readings on the sliding rail, graduated in tenths, and marked in units. The electric readings are both etched on the brass terminals and the corresponding plastic caps.
Dimensions (cm): 50cm x 27cm x 17cm
The very accurate measurement of electrical resistance. This instrument has a self-testing feature.
Very good. The exterior of the box is in good condition. There is slight corrosion on some of the internal metal components.
Cambridge Scientific Instruments Company, Cambridge
Date of Manufacture: c. 1910
Department of Physics, University of Toronto
“The Callendar & Griffiths Self-Testing Resistance Box” Scientific American Supplement Vol. 42 No. 1093 (December 12, 1896 pg 17475-17476.
Note: Although “Callender” now appears to be common, the spelling “Callendar” is accurate. The instrument is named for Hugh Longbourne Callendar (1863-1930), a professor at (in this order) Cambridge, McGill, University College London and Imperial College, London.