AVO multimeters consist of a line of electrical measuring devices developed in 1923 by Automatic Coil Winder and Electrical Equipment Co. Also known as avometers or just AVOs, the name comes from the multimeter's design, which measures direct voltage, direct current, and resistance. This translates into amps (A), voltage (V), and ohms (O). Many AVOs are for specific applications, such as use in the automotive industry.
General AVO Multimeters
The original avometer was built between 1923 and 1928 and measured seven ranges of current, voltage, and resistance. The DC version of the avometer lasted from 1928 until 1939 and featured 13 ranges. A version later appeared that could handle 22 ranges by utilising a push-button switch. In 1931, the Universal Avometer had 20 ranges and later extended to 34 and 36 ranges. These devices were replaced in 1939 by the Universal Avometer Model 40, which remained in production until 1986. It was a 36-range AVO with an automatic cut-off feature. Out of all the AVOs, the Universal Avometer Model 8 was the most popular, and much of its design remained unchanged from 1923 until 2008 when it ceased production. Newer versions are simply modern renditions of the original vintage AVO meter.
High Sensitivity AVO Multimeters
The Universal AVO meter, also called the Model 7, had 50 ranges and was a high sensitivity device specifically for servicing radios. It was produced between 1936 and 1986 and had ranges between 10A and 1000V, which could extend with current shunts. The Model 8 was also a high sensitivity multimeter with a range between 10A and 1000V, 2500V, or 3000V, depending on the model. The AVO Model 9 was basically the same as the Model 8, except it featured international symbols and markings for the switches. Its range was between 10A and 3000V.
Throughout the history of AVO products, numerous renditions were for particular industries. The AVOmeter Model 12 was for the automotive industry, for example, and the heavy-duty AVOmeter was a small device with a single switch built for the Great Western Railway company to help with signalling. It was also later sold to the public with a range of 10A to 1000V. The AvoMinor measured direct current, direct voltage, and resistance ranges by being plugged into a socket, and the AVO Multiminor replaced the original Minor series with a device that used a rotary switch to select ranges and functions. It had no automatic protection and was smaller for portability.