The balance of power
Today my first year classes did the magnetic flux density practical that I devised a couple of weeks ago. When I did it I got a pretty good result but didn't use exactly the same equipment as I gave to my students. The balance I used cost about £1000 the one they used cost £5. I think it makes a difference although the accuracy is supposed to be the same, the cheap version sometimes didn't register a difference. I remember reading in the manual (that I threw away) that the balance made a measurement every second or something, or was it 5s? The other problem was with the batteries, they don't last long when connected to 30cm of Nichrome. When I did it I only held the contacts on the wire for a second or two but when no force was registered some of the students held the contact in place a good deal longer. Another problem was those blasted magnets, they are so strong that its really hard to get them apart to place then across the metal yokes.
Oh well, back to the drawing board. I think I am going to modify the procedure by making some coils that fit over the magnets to increase the force. I could also then attach an extra length to act as the rheostat. An altogether neater solution. Better not make it too good though or my students won't be able to think of any improvements.