Sonotone A/B Battery Meter

This dual-scale battery meter carries the Sonotone name. It was one type of battery tester used in the mid to late 1940s to test both A and B batteries.

There were a number of battery meters that looked similar except for the company name on them. It appears one battery meter company designed the original meter and then manufactured and branded it for the various hearing aid companies that wanted one.
 

Normally (by convention) you touched the one tip of the probe to the positive terminal of the battery, and touched the "foot" (7-o'clock position) or the "knob" (12 o'clock position) to the negative terminal of the battery.

With this meter there was also a fold-out aluminum disc so that you could simply set the base (negative terminal) of the battery on to make battery testing even easier. This worked for batteries such as AA, C and D cells.

Notice that the probe tips are labeled "A" and "B". When you were testing an "A" battery, you used the "A" tip which was connected to the 2 volt range. Similarly, to test a "B" battery you used the "B" tip which was connected to the 50-volt range.

This tester had 2 scales—one ranging from 0 to 2 volts for testing "A" batteries, and the other ranging from 0 to 50 volts for testing "B" batteries. Notice that this was a 0-centered meter. Thus it didn't matter whether you put the probe to the positive or negative contact on the battery—the meter just read to the left or to the right depending on the polarity.