Alan; Lawrence -
I have made some measurements on a variable capacitor of the type shown in your last picture, Alan. With some approximation, it conforms to the law:
log[C] = mx + c where:
C is the resultant capacity of the capacitor;
x is the angle of enmeshment of the vanes (between 0° and 180°: as x increases, C increases);
m and k are constants.
In other words, it is a 'log law' variable capacitor.
The particular one I tested produced m = 0.01 and c = 1.18.
It was a SLW type with a Cmax of about 520 pF.
I looked this topic up in 'Radio Engineering', 1st. edition, (1932), by F. E. Terman. He lists this type of capacitor as a straight-line wavelength (SLW) type. He also gives sketch drawings of the 3 types: SLC, SLW and SLF. Upon appearance, the SLF has decidedly pointed ends; strange-looking beast: I'd certainly recognise one now if I saw one. Terman also gives a graph, dial rotation versus the three types, for comparison. Obviously, the SLC is a straight line. The SLW has a slight 'sag' (which is what I found on test), and the SLF gives a similar sag - but that sag is much more pronounced that that for a SLW type.
Finally, I found this on the 'Net, and although it makes for interesting reading, not all the comments made therein are correct.
http://www.eham.net/ehamforum/smf/index....ic=87948.0
Now I don't know why, but there is something about the laws & construction of variable capacitors that I find really fascinating (as you can probably tell by now) - perhaps it's the mathematics - or perhaps I'm just an anorak!
Anyway, it's late: I'm off to bed.
G'night!
Al.
Edited April 9th.: corrections to the derived capacitor law formula.
Al.
I have made some measurements on a variable capacitor of the type shown in your last picture, Alan. With some approximation, it conforms to the law:
log[C] = mx + c where:
C is the resultant capacity of the capacitor;
x is the angle of enmeshment of the vanes (between 0° and 180°: as x increases, C increases);
m and k are constants.
In other words, it is a 'log law' variable capacitor.
The particular one I tested produced m = 0.01 and c = 1.18.
It was a SLW type with a Cmax of about 520 pF.
I looked this topic up in 'Radio Engineering', 1st. edition, (1932), by F. E. Terman. He lists this type of capacitor as a straight-line wavelength (SLW) type. He also gives sketch drawings of the 3 types: SLC, SLW and SLF. Upon appearance, the SLF has decidedly pointed ends; strange-looking beast: I'd certainly recognise one now if I saw one. Terman also gives a graph, dial rotation versus the three types, for comparison. Obviously, the SLC is a straight line. The SLW has a slight 'sag' (which is what I found on test), and the SLF gives a similar sag - but that sag is much more pronounced that that for a SLW type.
Finally, I found this on the 'Net, and although it makes for interesting reading, not all the comments made therein are correct.
http://www.eham.net/ehamforum/smf/index....ic=87948.0
Now I don't know why, but there is something about the laws & construction of variable capacitors that I find really fascinating (as you can probably tell by now) - perhaps it's the mathematics - or perhaps I'm just an anorak!
Anyway, it's late: I'm off to bed.
G'night!Al.
Edited April 9th.: corrections to the derived capacitor law formula.
Al.
