MTFCA Forum

*** Disclaimer: This is an ultra nerded subject for electronic nerds. Anybody else are welcome to read, but bear over with me, thank you ***
Seing the electric specs for our trembler coils set me off to speculate:

We have an oscilator circuit here with a coil and a condenser so I wonder what the resonanse frequency would be for that circuit?

The formula for that is: According to the specs for a 1926 coil we have the primary (low voltage) coil of 0,0033 Henry (Henry is the unit for electromagnetic coils socalled inductance). The condenser we use to put in is 0,47 pF or 0,00000000000047 F. 2xPI is set to 6,283 for ease.

That give me a resonanse frequency of 4 MHz (4,041,355 Hz). This is for the isolated primary circuit. I am aware of the the secondary circuit may influence the inductance of the primary coil, but I do not know how much. I do not think it will change the resonance frequenzy several orders of magnitude, so we are talking MHz.

Given that the magneto gives out 16 Hz per revolution that will be 32.000 Hz or 32 kHz at 2000 RPM which is in the very high end of the Model T engines RPM interval.

So - as over 100 years of experience have showed - not an immidiate problem.

Have I missed something (I know there are electronic engineers around here - I'm just a mechanical engineer so what do I know

I have no idea how to calculate those values. I'd think you'd have to consider the inductance of both primary and secondary, the capacitor, and the input frequency as modified by the frequency of the vibrating points. The spark plug gap distance probably affects it, too. I do know that a T coil with the secondary discharging through a spark gap of about 1/8" can emit RF over a wide frequency range, easily blanketing .55 through 1.6 Mhz, and probably well beyond that. Connecting a Model T coil primary to a 6 V battery through a telegraph key and connecting the secondary to a spark gap with a 50 foot longwire antenna on the coil side of the gap and the other side earth-grounded will send a strong signal to any AM radio within at least several city blocks.

Microfarad, not picofarad.

Ahh Tom, you are right!! I should have read my documentation a little closer.

0.47mikroF = 0,000 000 47F so now the resonance frequency is 4000 Hz or 4kHz. Hm. And now that we are at it - after having slept on it, the 16 coils produce 8 cycles per revolution and not 16 (see below drawing), so the frequency of the output AC from the magneto is just 16kHz at 2000 rpms. The resonance frequency are reached at 500 rpms.
Ford Model T ignition - drawing curtesy John Regan.

So what can we use this information to? I have no idea, but I have wondered for long what the resonance frequency would be.

Hummm...I thought this was answered in the Dykes(1929,page 1082) Subsection on the Model T. "Magneto Speed and Voltage". At the time the the 16 coils produced 7.5cycles per revolution. Maxing out at 1,200 RPM.

A picofarad is a poor, wretched little thing.... quite tinny, actually, and just nearly nothing. Now a microfarad is another matter ... rich and woody... nothing tinny about it!

Michael,
I believe you forgot to divide the frequency by 60 in order to convert from cpm to cps. At about 450 rpm the model t mag output is at 60 hz.
Art Mirtes