Unloaded coil:

f1 = 146.54 kHz predicted, 146.45 kHz measured.
Lee = 51.3mH; Les = 59.3mH; Ldc = 74.1mH;
Cee = 17.2pF; Ces = 20.0pF; Cdc = 50.1pF;
Q = 60; Voltage gain = 68.8; 
Measured on 22nd October 2000.

Toroided coil:

f1 = 85.27 kHz predicted, 86.7 kHz measured.
Lee = 71.6mH; Les = 73.4mH; Ldc = 75.4mH;
Cee = 46.3pF; Ces = 47.5pF; Cdc = 70.9pF;
Q = 164; Voltage gain = 168.5; 
Measured on 13th November 2000.

See [3] for images of the test setup.

Terry's gradient measurement technique returns the peak gradient across an interval of approximately 1", the approximation occuring because the number of turns per inch happens not to be an integer. This causes an apparent jitter in the gradients, which we take account of by arranging to predict the gradient using exactly the same interval of turns at each of the 30 measurement steps.

An allowance of 0.12 volts was added to each reading to take account of the forward voltage drop across the peak detector diode.

The measured and predicted voltage profiles, aligned through the base input current, are compared in the graph below. The jitter occurs due to the necessity to select a probe placement across between 32 and 35 turns, as mentioned above.

The accumulated top voltage of the measurements is 69.72 volts, which is 1.6% higher than the predicted 68.6 volts.

The predicted gradient peak at around 8" coincides nicely with measurements in both position and amplitude. Below the gradient peak the voltage rise is concave and in this region the measured gradients are a little below the predictions. This is probably due to a reduced external capacitance resulting from the partial ground plane used in this test setup - the predictions were based on a continuous groundplane. The extra Cext in the latter case would generate a slightly higher dV/dx in this region. This seems to have a knock on effect all the way up the coil, giving a slight skew to the measurements with respect to the predicted curve. The measured gradient peak is probably slightly above the predicted peak as a result, say 9".

The measured and predicted voltage profiles are aligned this time through the top voltage, as a base current reading was unavailable.

This shows excellent agreement between measured and predicted values. As expected from theory, the addition of a topload has enforced a more uniform voltage gradient - a beneficial effect when operating a coil near to its stress limits.

When the above gradient profiles are bezier smoothed and combined, we obtain the composite graph below.

It is clear in the toroided case that the gradient peak is located higher up the coil, thus further separated from the induced EMF gradient peak generated by a typical primary coupling.

These precision gradient measurements provide unequivocal experimental evidence of the point of inflection occuring in the voltage profile in the lower half of the coil. Accuracy of both the measurement technique and the software model is demonstrated by our ability to predict the variation due to 'turn quantisation' of each reading.

As a corollary these results also constitute good indirect evidence of a current maximum elevated above the coil base.

[1] See http://hot-streamer.com/TeslaCoils/MyPapers/NSVPI/NVSPI.htm for details of Terry's measurement method.
[2] See pn1710 for examples of voltage and current profiles.
[3] Images of the test setup can be found under http://hot-streamer.com/TeslaCoils/Misc/PaulNich/ProfilerPics/