(very) small current ammeters - usage of? - Page 1
Forum section: electronics / beginners

author: cspxo
author_rank: Newbie
timestamp: October 10, 2013, 07:37:09 pm
content: I have two meters that I am curious about. The first is a meter with a small (approx 3cm dia. bezel) round face, the needle at rest points vertically with a red sector to the left and a green sector to the right. It has the word NULL printed on it and +/- 500uA. I believe it may have come from a military field radio. The second is an old (1950s?) meter reading to 10 mA made by E.M.I. Service Ltd. of Hayes, Middlesex.My question is what are such meters used for? Apart from the obvious answer, measuring current! Can anyone give me specific information or examples please? I have googled but the interwebs greatest failing obstructs my quest for knowledge in this instance, all I get back are Chinese vendors of modern equivalents with specs but no useful information.I am self-taught/learning about electronics from books which is great for theory but apart from very simple example circuits I am finding real-world applications e.g. descriptions of actual products working circuits very hard to find.Thanks for reading.--Stephen

author: niflheimer
author_rank: Contributor
timestamp: October 10, 2013, 07:41:42 pm
content: The first one is a null-current indicator used in things like resistance bridges and so on . It was a cheaper solution compared with the optical null detector ( which was flimsy as hell too ) . They both show when the current on one branch of the circuit is equal with that on the other branch.I dont know what it was used for in that application though - maybe some of the older guys around here can tell us.

author: Stonent
author_rank: Super Contributor
timestamp: October 10, 2013, 07:52:31 pm
content: I know 50uA ammeters are used as the displays for Geiger counters. The scale is changed to counts per minute but still current driven.

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 10, 2013, 08:15:17 pm
content: Before the days of digital displays and ADCs, most meters were, at their heart, small-current DC ammeters. You can google "D'Arsonval", or "D'Arsonval/Weston" for details and an explanation of the theory of operation. They're still used for most moving-needle analog meters, but those are getting harder to find these days.Additional circuitry would be used to adapt these to measure voltage, resistance, different ranges of current, etc. In a radio, they might be used to measure signal strength, or maybe as an antenna tuning aid or SWR indicator (just a wild guess). Different scales would be added to the dials for various purposes.

author: cspxo
author_rank: Newbie
timestamp: October 11, 2013, 11:16:16 am
content: Thank you for your responses.AG6QR: I have previously come across information about D'Arsonval/Weston. When I look at 'vintage' or salvage parts, I suppose what really interests me is the history and detail of the kinds of machinery or equipment that preceding generations might have worked with.Stonent: That is an interesting example.niflheimer: I wonder if a null detector is the same thing as a 'magic eye' I have seen referred to in descriptions of old domestic radios?I am very interested in the 'history of electronics' as the appliances and devices we are all so familiar with form a sort of backdrop for the social history of the last century. Today's teenagers may not know what cassette tapes are, but my Grandfather was a boy in a world that didn't have broadcast radio for example.

author: dfmischler
author_rank: Frequent Contributor
timestamp: October 11, 2013, 12:34:18 pm
content: Quote from: cspxo on October 11, 2013, 11:16:16 amThis is what I know of as amagic eye tube. When I was a kid my Dad had a Stromberg Carlson radio that had a magic eye for accurate tuning.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 11, 2013, 02:03:30 pm
content: Quote from: cspxo on October 10, 2013, 07:37:09 pmThe number of devices which may have used a centre zero meter is almost countless.In this case,it may be an "idiot meter" to allow non-technical staff to tune a transmitter.It would have additional circuitry associated with it,so that it could indicate which direction you were off tune.The second meter was a pretty much "run of the mill"meter,probably used with a shunt to allow it to read higher currents.It is unlikely to have been part of a multimeter,as they used considerably more sensitive movements.Similar analog meters are still widely available.

author: rsjsouza
author_rank: Super Contributor
timestamp: October 11, 2013, 02:59:20 pm
content: Quote from: Stonent on October 10, 2013, 07:52:31 pm50uA was also widely used in several good quality analog meters.Professional wheatstone bridges used middle center galvanometers (that is how I called them) - one example ishere.

author: G7PSK
author_rank: Super Contributor
timestamp: October 11, 2013, 03:20:42 pm
content: Martin Lorton recently did a Video on a Heath voltage reference where he describes how a null meter might be used to find the voltage of a source using the reference.