Roller-Smith Bridge Ohmmeter

There is nothing digital about a vintage Roller-Smith bridge ohmmeter.  It can measure from 0.5 ohms to 50,000 ohms using 4 ranges.  This is a beautiful antique test instrument.  It has an attractive hardwood case, leather handle, and quality construction. 

Roller-Smith bridge ohmmeter
Roller-Smith bridge ohmmeter

The meter operates on 3 vdc, which can be supplied using two D-cell batteries or a portable external dc power supply connected directly to the battery terminals.

Usage is very simple.  Connect the resistance to be tested to the terminal posts, select the appropriate range via plug socket connection, push the Test button and rotate dial until meter nulls at zero.  At this point, the bridge is balanced, and ohms is read by multiplying the dial scale reading by the plug-socket range.

Dial low (resistance higher)
Dial low (resistance higher)
Dial high (resistance lower)
Dial high (resistance lower)

Bridge-type equipment has always been very accurate, and this is no exception.

The following photos demonstrate accuracy on all four ranges, with resistance verified by Fluke DMM.

Meter swings left or right while the bridge is unbalanced.  Swings to the left (dial too low, attached resistance higher.)  Swings to the right (dial too high, attached resistance lower).

Range X1 — “15 ohms” (16.6 ohms Fluke)


Range X10 — “330 ohms” (320 ohms Fluke)


Range X100 — “2.2K ohms” (2350 ohms Fluke)


Range X1000 — “15K ohms” (15,830 ohms Fluke)