mystery box - Page 1
Forum section: electronics / beginners

author: p.larner
author_rank: Frequent Contributor
timestamp: November 25, 2023, 07:23:34 am
content: i just got this for 99p on ebay,any idea what it is and what its used for?,i just bid for the hardware binding posts etc,

author: magic
author_rank: Super Contributor
timestamp: November 25, 2023, 08:07:13 am
content: Older than meSome sort of resistance measurement bridge with bulit-in reference resistance decade box?

author: Gyro
author_rank: Super Contributor
timestamp: November 25, 2023, 10:38:32 am
content: Yes, it's a Gambrell Wheatstone bridge. The terminals, top center, for the Galvanometer (null detector) and energizing battery (just a few volts).As magic says, the four dials across the bottom are effectively a decade resistance box, with the multiplier and divider knobs on the other arms of the bridge to allow resistance measurement over a large range of values. The buttons at the bottom temporarily apply the battery voltage and connect the galvanometer (you don't want to damage it (sensitive moving coil) when the bridge is way out of balance.It would be well worth trying it out before pulling apart. Just use a dmm on mV range as a null detector and maybe 2-4V on the battery terminals. It is probably very accurate (compared to, say, a 3 1/2 or 4 1/4 digit dmm. It will be full of decently precise wirewound resistors and good switch contacts (that might need a clean if it's intermittent.

author: Dan123456
author_rank: Regular Contributor
timestamp: November 25, 2023, 10:41:33 am
content: https://www.radiomuseum.org/r/gambrell_resistance_bridge_patt_2086.htmlNot sure if it is the exact same model but googled the patent number and got the aboveAs Magic guessed, it’s a resistance bridge… I have no idea what that means though

author: p.larner
author_rank: Frequent Contributor
timestamp: November 25, 2023, 11:01:45 am
content: thanks,it will be good if it arrives,ive had it a few times on ebay where the winning bid is lower than the seller was expecting,only to be told the item has got destroyed etc,so it can be relisted at a later date,i am not holding my breath!.

author: Ian.M
author_rank: Super Contributor
timestamp: November 25, 2023, 12:26:16 pm
content: Better photos of an older model of the same #2086 bridge here:https://www.radios-tv.co.uk/community/in-the-workshop/vintage-resistance-box/including interior photo good enough to figure out its internal connections.It appears to be usable as a resistance substitution box between the lower ones of its X and INF terminals, so even if you ignore its resale value, and have no use for a Wheatstone bridge, it would be foolish to strip it for parts.

author: donlisms
author_rank: Frequent Contributor
timestamp: November 25, 2023, 02:00:26 pm
content: I would suggest that spending at least a little time understanding how it works could be a good way to get familiar with bridge circuits in general. That could be helpful when you decide you want to build a temperature sensor bridge circuit and other such things. It's a good way to think about dividers and using ratios in general.

author: WimWalther
author_rank: Regular Contributor
timestamp: November 25, 2023, 05:47:11 pm
content: Sorry to do this to you, but you may need to find another source of vintage hardware..That is a very high-quality instrument, real old-school lab grade stuff. Unless something terrible has gone on with it internally, it would be a shame to scrap it for knobs & posts.As others have said, at least learn how it works and how to use it before making any decisions. This is a great opportunity to further your knowledge of electronics in general and the development of precision metrology equipment.Hope it shows up - it's a very nifty piece!

author: magic
author_rank: Super Contributor
timestamp: November 25, 2023, 05:52:24 pm
content: Sorry to disappoint but if I were you I'd hope that it doesn't show up...

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 25, 2023, 06:09:37 pm
content: I suspect you can sell it on this forum for enough to cover your costs, brand new binding posts, several knobs and more. Cost of shipping might be a barrier, but otherwise is virtually a sure thing. That is if 99p means 99 pence. If it was 99 pounds you might still do ok.As others have said, it would be a shame to scrap a fine instrument like this which can still be used to make higher accuracy measurements than todays garden variety DMMs.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 19, 2023, 09:24:52 pm
content: Thought i would give an update in this,its in beautufull condition in a nice dovetailed oak box,the resistor in the first position top left was open,marked 10000,so fitted a 10k 1/2w metal film resistor,i have just brought a Solartron 7075 Schlumberger Digital Voltmeter,will this box help to test this meter,looks like a 9 digit unit?,thanks.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 19, 2023, 10:53:40 pm
content: Do you mean that there was NO resistor on the switch in this Position? Was there any sign the resistor was there?? Your choice of resistor, in any case is probably way off. There may not have been any resistor in this setting.You need to apply a battery voltage across the battery terminals, I would think that voltage would be maybetwo "D" cells.Perhaps someone knows the correct voltage, it should be in the range of 2 to 4 volts as has been previous recommended and the exact value should not matter much.Connect your voltmeter across the Galvanometer terminals. They want you to connect a mechanical center Zero galvanometer here, but you probably do not have one, Your digital voltmeter should be OK.Then put a resistor in the "X" terminals and turn the knobs to the resistance of your "unknown" resistor. The Voltmeter should be very close to zero when you have the correct amount resistance of ohms on the box.Those resistors are very expensive things and the contacts of the switches are also very good, please do not damage them. Also the solder in any of these things is special, if there actually is solder.If you clean any terminals, be careful and use only good products, The terminals are probably very low resistance pure copper. A real calibration person would also only use pure copper wire connectors. again, no solder even.These things are extremely accurate and do not drift much over 100 years. The probably could pass calibration in a laboratory even at their age.This is a Wheatstone Bridge, Please read about them. It used to be the best thing for accuracy and is a Laboratory Instrument. It is probably more accurate than any new multimeter until you get near maybe ten thousand USD in purchase price.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 19, 2023, 11:38:39 pm
content: If you set it up for Divide by ten and Multiply by ten on the top switches then the four digits on the bottom switches should be the value of your resistor in ohms.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 19, 2023, 11:44:08 pm
content: Well the wire wound resistor i was refering to was oc,its impossible to find the break to repair it,the wire is thinner than a hair+it must consist of 100+turns,,hence my bodge,at least now it works,i was more interested on my new purcase the solatron 7075,its still in the post,can anyone tell me to how many decimal places they can read too?,it will measure micro ohms i think and micro volts,thought it would be handy for helping locate shorts in power rails?.