Huge thanks to all the people who watched and commented on my latest project video about the watt balance
I got a lot of great feedback and interest including from several of the scientists at NIST where they're working on full-size
laboratory-scale watt balances
this video is a follow-up to that to show some of the details about how the balance was made and
Answer a few of the more popular questions that popped up in response to that video if you haven't seen it yet
There's a link below in the description
the main parts of the watt balance were made from cherry which is a hardwood that's domestic to the US
I started with a rough sawn board which I resawed on the bandsaw
Then I planed it to the correct thickness
The uprights were cut on the bandsaw using a printed template I drew on the computer
I sanded all the parts then put a chamfer on the edges using a router table
for the knife edge pivot I used a piece of brass bar filed to a semi sharp Edge on two sides
this was glued into a kerf in the center of the balance beam
I also glued two small pieces of brass on the insides of the uprights for the knife edge to pivot on
The uprights were connected to the base using washer head wood screws
The platforms were turned on the lathe from a piece of an elm tree that someone had cut down and left beside the road
First I turned the outside shape then I drilled a hole in the bottom
Finally it was parted off
This process was replicated for the other platform
The coils on the platform's consists of about 3000 turns of 36 gauge Magnum wire
I set up a winder using my drill and a few clamps
This allowed me to control the speed of winding easily and stop when necessary
I also connected a Hall effect sensor to an Arduino and wrote a quick script to count the turns
Each time the magnet passed the sensor, the program would send an update over the serial connection giving the turn count and speed I?
Also stopped every so often to pot the coil in Epoxy to hold everything together
Winding these coils was definitely the hardest part of the entire project
Working with wire this then is difficult, so if you try this yourself just be patient
If you get in a hurry you'll break the wire and have to start over like I did. Twice.
Just for reference, each coil took about an hour and a half to finish
The hangers for the platforms were made from brass bar stock bent in a bench vise
Not the most elegant way to do this, but the results weren't too bad
These were glued in with Epoxy
Once assembled, everything got a few coats of Danish oil to protect the wood and make it look fancy
I connected everything to the circuit board using 24 gauge Magnet wire not for any other reason other than I thought it would look nice if
I did it over again
I would use a much lighter gauge wire to avoid imposing any extraneous forces on the balance beam from the springiness of the wire
One commenter Don Conlee had some good ideas about moving the electrical components off the balance beam to eliminate this issue entirely
he suggested swapping the permanent magnets into the platform so that the coils are stationary and
Using a mirror on the beam with a stationary laser rather than having the laser attached to the beam itself
The circuit for this is pretty straightforward, and you can pull the diagram from the paper linked in the description
I put everything on some perf board and connected it up with Kynar wire
the circuit hooks up to a national instruments data acquisition box which allows the software to
Communicate with the watt balance, and I just saw that NIST is working on some new software to add features and improvements
That's a very brief summary about how the watt balance was built again this design was based almost entirely on a paper
prepared by some of the engineers and scientists at NIST as well as
The University of Maryland which I have linked below and I just want to say huge thanks to them for making outreach a priority
and designing this cool demonstration now for a few of the more common questions
No. 1: Did you see Veritasium's video on the Silicon sphere?
Yes I did and he did an awesome job like he always does with physics subjects
I'll link that video below for anyone who hasn't seen it
No. 2: How accurate is it? From my testing I was coming within around 10 percent for all the measurement I took
I didn't do a thorough analysis of the error
but I think I could get a lot closer with a more accurate way of measuring current and
also by reducing the influence of the wires connecting the various components of the balanced
No. 3: Doesn't your value for gravity affect the measurement?
Yes, this is something I kind of gloss over
But should have explained a little bit better since the Earth's gravitational acceleration is one of the variables needed to measure
Mass using the watt balance it has to be measured very accurately
Just like all the other inputs Earth's gravity changes by location
But also in time due to changes in ground water and other fluctuations
So it needs to be measured each time you use the watt balance
Metrology labs have ways to do this accurately?
But I don't so I just use the international gravity equation in the video which
Approximates the gravitational acceleration in your location based on latitude
No. 4: Wait, wait, wait, the volt and the amp use the kilogram in their definition, so how can they be used?
To redefine the kilogram to be based on a physical constant
This is a great question and it's one I intentionally
Avoided to keep the video relatively simple for anyone to understand the redefinition of the kilogram requires
fixing the value of Planck's constant by using quantum physical effects to measure
Electrical quantities with a mass traceable to the Ip k it gets a little bit complicated
so if you want to know more about that the paper in the description that I linked below does an awesome job of explaining the
process in more Detail
Finally I also got a lot of really nice comments about the drawing I did during the intro sequence
So I made a separate video about that and I also
got some prints of that drawing made and put on my website in case you'd like to hang one on your wall and
By the way, I finally have t-shirts on the website as well, and they're very very soft and comfortable
More videos are coming soon huge thanks to the fans making that possible by supporting the channel on Patreon
Thank you for watching and let me know what you think