The Textbook Behind the VIC, 4: the choke under a blow

Part 4 of 6 in The Textbook Behind the VIC: 1 · 2 · 3 · 4 · 5 · 6

What Meyer wrote

Purpose: To utilize Resonant Charging Chokes to aid amp restriction.

— Voltage Attenuation Circuit

Component arrangement of said Voltage Intensifier Circuit 9XA as to 20YA retards or prevents amp flow.

— Dual Voltage Resonant "Q"

The phrase that runs through all of Meyer's writing is that the VIC puts voltage on the water while holding back the current. To anyone thinking in steady currents that sounds impossible: a voltage across a conductor drives a current through it, and Ohm's law says how much. Fleming's answer is that Ohm's law is the slow case.

What Fleming wrote: the electromotive blow

If between the ends of a conductor a difference of potential is created which is brought about slowly, the result shows itself in a current in the conductor… If, however, the difference of potential is created with great suddenness, the resulting electric flow is less determined by the true resistance, and more by the inductance of the conductor. In this case we have the phenomena of impulsive discharges.

— Fleming, §8 of Chapter V, pp. 399–400 (part 23)

A conductor of sensible inductance can no more have a current of finite magnitude created in it instantaneously than a body of sensible mass can have a finite velocity instantaneously given to it. In both cases there is an immense resistance to very sudden change of condition.

— Fleming, p. 400

Fleming's illustration is one Meyer, who wrote about the "thermal explosive energy" of his gas, would have liked: gun-cotton laid on a stone slab and simply lit burns away and leaves the slab whole; fired with a detonator, the same charge is so sudden that the air cannot get out of the way, and the slab shatters. Inductance is the electrical version of the air's inertia.

Lodge's alternative path

Fleming then describes Oliver Lodge's experiment of the "alternative path". A Leyden jar discharge is offered two routes: a thick metal wire of a fraction of an ohm, or a small air gap of many megohms. The discharge takes the air gap:

…although there is a divided circuit open to the discharge, one branch of which measures hundreds of thousands of ohms or megohms and the other only a small fraction of an ohm, it nearly all goes by the route of higher resistance… The electromotive impulse meets with such resistance owing to the electro-magnetic inertia of the circuit that it rebounds and cracks through the air.

— Fleming, pp. 401–402 (part 23)

This is the principle the telegraph engineers used for lightning protectors, and it is the principle of Meyer's chokes: to a fast pulse, a coil with a large inductance looks like an open circuit, so the pulse's voltage stands across whatever else is in the path — here, the water cell — rather than driving a current through the coil.

How much restriction, in numbers

Fleming's rule for the first instant is that current can only grow at the rate the voltage divided by the inductance allows. Take Gabel's measured chokes on the core, 1.26 H and 1.14 H in series (2.40 H, VIC coil readings), and suppose the whole of Meyer's 5,070 V (from his 1987 pencil pages) were across them at the start of a pulse. Current could then grow at no more than

5,070 V ÷ 2.40 H ≈ 2,100 A per second — about 2 milliamps per microsecond.

A 50-microsecond pulse cannot push more than about a tenth of an ampere through those chokes, whatever the resistance of the water. That is amp restriction, and it is Fleming's §8, not a mystery. It is an upper bound under a stated assumption, not a measurement of a running cell; the capacitor, the leak and the secondary change the details.

Meyer added one more restriction deliberately. He wound the chokes of resistive wire and said so: "The resistive value of said Charging Choke (43) acts as a resistor, preventing amp flow still further" (Dual Voltage Resonant "Q"). Part 5 is about what that resistance does to the ringing.

A warning from the same chapter: iron and fast pulses

Dr. Lodge has called attention to the fact that a "choking" coil having a core of divided iron and wound over with many turns of wire does not add to the apparent self-induction of a circuit discharging a Leyden jar… eddy currents are set up even in a core of finely-divided iron, and… under sufficiently rapid alternations, are confined to the surface of the core.

— Fleming, Chapter V (part 24)

The sixteen-fold gain from the iron in part 1 was measured at 100 Hz. For very fast edges the iron contributes less, because the eddy currents keep the flux out of it. A VIC choke's inductance should be measured at the frequencies the pulse edges contain, not only at the pulse repetition rate.

What to measure

• The current through the cell during a pulse, with a small non-inductive sense resistor and a scope. Compare its peak with voltage ÷ inductance × pulse width.
• The choke's inductance at 1 kHz, 10 kHz and 100 kHz. If it falls steeply, the core is not following the edges.
• The voltage across the cell and across each choke at the same instant. Lodge's experiment says the fast part of the pulse should appear across the cell.

Next: part 5, ring or dead-beat.