Taut band milliammeters: properties? - UK Vintage Radio Repair and Restoration Discussion Forum

Skywave · 11th Dec 2015, 7:02 pm

Yes, I have heard about such milliammeters - but have never owned one nor used one - until now; a recent acquisition. It's 1.5 mA FSD; internal resistance 6 Ohms. My previous experience with analogue meters has been limited to moving coil and moving iron types.

So I'm really quite ignorant about taut band milliammeters. But I know that here I stand a good chance of enlightenment. Here's one Q. for starters: what are the essential differences, especially from a usage viewpoint, of such a meter, compared to a moving coil type?

All replies will be much appreciated.
Thanks.

Al.

TonyDuell · 11th Dec 2015, 8:02 pm

As I understand it they _are_ a moving coil movement. Similar coil and magnet assembly to a 'normal' one. The difference is the pivots and restoring springs. The normal moving coil meter has the coil pivoted on jewelled bearings with hair springs to (a) provide the restoring force and (b) to conduct the current to the coil. A taut band meter has the coil held by torsion wires (one each end, where you'd expect the pivots to be) which support the coil, allow it to pivot (there are no jewelled bearings), provide the restoring force (the wires act as torsion springs) and conduct the current.

The main advantage is that they are mechanically more robust for a given fsd and they can be made a lot more sensitive (no friction in the pivot to worry about).

For general use there's probably not a lot of difference.

WME_bill · 11th Dec 2015, 8:04 pm

Taut band meter movements.
Differs from moving coil in that there are no hair springs or jewelled bearings. The coil is held in the magnet gap by a thin band of phosphor bronze or similar material top and bottom, which carry the current into and out of the coil and provides the torque to resist the action of the current though the coil.
Simplifies the construction of the movement, but requires a stronger coil former to resist the tension of the band. Less friction as no bearings, so possible to make the movement more sensitive, giving permitting lower full scale currents.
The torque from the band will be temperature sensitive, as with the traditional coil springs, so compensation is required, by thermistor or buffer resistors of selected materials such as manganin.
Use it just like any other meter.
I suspect the centre pole style which Taylor produced, and used by Avo after they bought Taylor was particularly suited to taught band construction. Does anyone know?
Usually used only for moving coil meter, though it could I suppose be used for moving iron, though the repulsion style of moving iron would be rather out of balance. I have never seen one.
wme_bill

G6Tanuki · 11th Dec 2015, 9:36 pm

Essentially, it's all in the suspension - no 'snail springs' or bearings to support the moving coil bit.

I generally associate taut-band movements with some of the technically-nice-but-also-pleasantly-cheap multimeters imported from the USSR by Techical&Optical Equipment [TOE] in the 1970s - though I'm also aware that taut-band meters were also used in some military aircraft, as for a given level of sensitivity they can be made rather more robust/vibration-resistant than spring-and-bearing types.

Radio Wrangler · 11th Dec 2015, 9:41 pm

There were some cheaper taut band movements around which suffered a bit from hysteresis and short life.

There were also some very good ones

David

Skywave · 11th Dec 2015, 11:28 pm

Thanks guys.

Just for reference, the meter scale plate states that is made by Pye Unicam. The metal box in which this meter is situated also states Pye Unicam. The electronic components on the pcb in the box have a distinct mid-1970's look about them and the mains lead (which I am sure is original) has cores that are brown, blue and yellow/green. All the fixtures are BA sizes.

Al.

MotorBikeLes · 11th Dec 2015, 11:54 pm

Did not the Unicam "Scalamp" meters have the mirror suspended by taut wire mechanism?
We used Kent Mk1 potentiometer recorders which were supplied in the late '40's. They had a very simple taut wire suspended paxolin pointer that was trapped and assessed every few seconds by an ingenious mechanical arrangement.
I won't go into that (so damn complex) but the service engineers could easily replace the movement on-site if required. I think a set of three suspensions of different sensitivity were either supplied or available.
Not sure a moving coil movement could be replaced that easily.
I had one of the USSR test meters supplied by O&T, don't know why I parted with it. Very good for high value resistance measurements, and could be extended with a 35v (?) external battery.
Les.

11th Dec 2015, 7:02 pm	#1
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Taut band milliammeters: properties? Yes, I have heard about such milliammeters - but have never owned one nor used one - until now; a recent acquisition. It's 1.5 mA FSD; internal resistance 6 Ohms. My previous experience with analogue meters has been limited to moving coil and moving iron types. So I'm really quite ignorant about taut band milliammeters. But I know that here I stand a good chance of enlightenment. Here's one Q. for starters: what are the essential differences, especially from a usage viewpoint, of such a meter, compared to a moving coil type? All replies will be much appreciated. Thanks. Al.

11th Dec 2015, 8:02 pm	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Taut band milliammeters: properties? As I understand it they _are_ a moving coil movement. Similar coil and magnet assembly to a 'normal' one. The difference is the pivots and restoring springs. The normal moving coil meter has the coil pivoted on jewelled bearings with hair springs to (a) provide the restoring force and (b) to conduct the current to the coil. A taut band meter has the coil held by torsion wires (one each end, where you'd expect the pivots to be) which support the coil, allow it to pivot (there are no jewelled bearings), provide the restoring force (the wires act as torsion springs) and conduct the current. The main advantage is that they are mechanically more robust for a given fsd and they can be made a lot more sensitive (no friction in the pivot to worry about). For general use there's probably not a lot of difference.

11th Dec 2015, 8:04 pm	#3
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Taut band milliammeters: properties? Taut band meter movements. Differs from moving coil in that there are no hair springs or jewelled bearings. The coil is held in the magnet gap by a thin band of phosphor bronze or similar material top and bottom, which carry the current into and out of the coil and provides the torque to resist the action of the current though the coil. Simplifies the construction of the movement, but requires a stronger coil former to resist the tension of the band. Less friction as no bearings, so possible to make the movement more sensitive, giving permitting lower full scale currents. The torque from the band will be temperature sensitive, as with the traditional coil springs, so compensation is required, by thermistor or buffer resistors of selected materials such as manganin. Use it just like any other meter. I suspect the centre pole style which Taylor produced, and used by Avo after they bought Taylor was particularly suited to taught band construction. Does anyone know? Usually used only for moving coil meter, though it could I suppose be used for moving iron, though the repulsion style of moving iron would be rather out of balance. I have never seen one. wme_bill

11th Dec 2015, 9:36 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: Taut band milliammeters: properties? Essentially, it's all in the suspension - no 'snail springs' or bearings to support the moving coil bit. I generally associate taut-band movements with some of the technically-nice-but-also-pleasantly-cheap multimeters imported from the USSR by Techical&Optical Equipment [TOE] in the 1970s - though I'm also aware that taut-band meters were also used in some military aircraft, as for a given level of sensitivity they can be made rather more robust/vibration-resistant than spring-and-bearing types.

11th Dec 2015, 9:41 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,894	Re: Taut band milliammeters: properties? There were some cheaper taut band movements around which suffered a bit from hysteresis and short life. There were also some very good ones David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

11th Dec 2015, 11:28 pm	#6
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Taut band milliammeters: properties? Thanks guys. Just for reference, the meter scale plate states that is made by Pye Unicam. The metal box in which this meter is situated also states Pye Unicam. The electronic components on the pcb in the box have a distinct mid-1970's look about them and the mains lead (which I am sure is original) has cores that are brown, blue and yellow/green. All the fixtures are BA sizes. Al. Last edited by Skywave; 11th Dec 2015 at 11:35 pm.

11th Dec 2015, 11:54 pm	#7
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Taut band milliammeters: properties? Did not the Unicam "Scalamp" meters have the mirror suspended by taut wire mechanism? We used Kent Mk1 potentiometer recorders which were supplied in the late '40's. They had a very simple taut wire suspended paxolin pointer that was trapped and assessed every few seconds by an ingenious mechanical arrangement. I won't go into that (so damn complex) but the service engineers could easily replace the movement on-site if required. I think a set of three suspensions of different sensitivity were either supplied or available. Not sure a moving coil movement could be replaced that easily. I had one of the USSR test meters supplied by O&T, don't know why I parted with it. Very good for high value resistance measurements, and could be extended with a 35v (?) external battery. Les.