Direct "Literal" Demonstration of the Effect of a Displacement Current, страница 2

Our apparatus is based on a large capacitor with two 80-cm-diam aluminum plates, held apart with a 10-cm spacing. The lower plate is grounded, and the upper plate, has a banana plug in the top so that, the connection to it may easily be opened and closed. The capacitor is driven from a small powdered iron torus transformer (Ferroxcube 400 T 750) which is wound with a ratio of 50 primary to 1000 secondary turns. This matches the capacitor to a 600 Q audio oscillator

Nolcs and Discussions

(Hewlett Packard, Model 200CD or equivalent,) , so that there is a voltage of one hundred volts

across the capacitor at about '20 kHz.

The magnetic induction pickup coil is in thc form of a torus and is made as follows: A 238-cmlong, 1.27-cm-radius rubber vacuum hose is wound with 14 000 turns of No. 38-gauge Formvar insu- lated wire. This can be donc by threading a dowel through the tubing and using a hand drill as a rotating device. The wire is glued onto the tube with silicone RTV cement just to fogilitate handling. The wire is insulated and protected against

abrasion by a larger thin rubber hose, (1.27-cm inside, radius, 4-mm thick wall) which is slit lengthwise, slipped over the coil and taped securely with mylar tape.

Copper shielding foil is added as shown in Fig. 2(b) ; the tubing is bent into a circle, excess end

rubber is cut off, and the tubing plus shielding is

attached to a piece of plywood backing in order to hold it rigidly. The plywood piece hos a large

hole cut in the middle. and has a handle as shown

in Fig. 2(a). At the junction of the handle. and the torus, the two ends of the toroidal coil are brought to a 1:1 audio-output transformer with

secondary center tapped.

                              (b)    

FIG. 2. (a) Toroidal coil assembled on plywood or phenolic bourd brace. (b) Cross section A—A of toroidal coil showing: (l) rubber tubing form; (2) wire windings; (3) outer insulating tubing; (4) Mylar tope to insulate overlapping portion of (5) cepper shielding foil • about 0.0025 cm thick; (6) plastic hose or coble clamp.

AJP Volume 42 / 247

Notes and Discussions

Probably any audio interstage transistor transformer will do here, but ours was made from a powdered iron Ferroxcube 528T 500 toroidal form wound with 400 turn primary and 400 turn center-tapped secondary. In order to be, conservative about stray pickup, the secondary was wound with 20() turns right-handed, and 200 turns overwound left-handed, and the starting points of these two coils were used as the common secondary center tap which was connected to the shielding foil. The secondary of this transformer is led by two shielded RG/59U or equivalent cables to the differential input amplifier terminals of a laboratory type oscilloscope.

It is important to notice that this type of connection, from the coil to the oscilloscope, is essential to the elimination of unwanted stray pickup Voltages. The desired signal is push-pull or differential in na.ture. The small 1 :1 transformer helps to reject the common-mode pickup voltage. Common-mode, rejection is improved by using an oscilloscope (in our case a Tektronics 503) with a good differential amplifier input with sensitivity of about 1 mV/cm. Thc shielding of the coil is important, and it also helps greatly to incltlde the small transformer mentioned within the copper foil shielding. The center tap of the transformer secondary is grounded to the shielding.