Miscellaneous and Wondrous Things in E&M
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### Lecture Description

The subject of ELECTRICITY & MAGNETISM possesses an endless abundance of enchanting things to explore and many things we do raise rather troublesome questions.

A - Here reside two 110-volt - incandescent - clear-glass - lamps.
They are identical in their physical properties. We energize them together. They look equally bright. But one of them - we say :— is energized on 110 volts AC and the other on 110 volts DC. How can we tell them apart? Answer: Approach one of them with a strong magnet. This we did. And what did we witness? The filament. oscillated in a pretty sweeping pattern. This lamp is clearly energized on 110 volts AC. The reason is obvious. Now we ask: what will the filament in the other do when that lamp is energized on 110 volts DC? Try it.

B - A solenoid is wound on a clear lucite plate. Iron filings are
sprinkled on the plate near the wire loops. They fall with abandon and no special way- We now energize the coil on a 6-volt battery and presto! The filings take up a beautiful pattern in and around the loops of the coil. The field of such a solenoid is quite like the field of a bar magnet. An interesting question arises: if we explore the field IN a solenoid mathematically we find the field strength on the axis of the coil in independent of the radius of the coil. That means that a small tiny coil and a big big coil have the same field strength along the axis - which is hard to believe. But it is true!

C - We have two lamps in series - one a 10-watt lamp - the other a 100-watt lamp. We energize the pair on 110 volts 60 cycle AC. Question: What do we see? Or - what happens? That is - do they both light up? Does only ONE light up? Or what? Maybe none lights up! We see that the small wattage lamp lights but the other does not. What must we say of this?

D - The Horn Gap or Jacob's Ladder: A pair of stiff metal conductors are fixed to a platform with their lower ends closer than their upper ends. We. connect the pair to a high voltage such as an induction coil. The spark bridges the gap at the lower end since the separation is only an inch or so. We see that the spark is making some effort to climb the ladder. It does - for a few inches. How can we have it climb all the way up to where the separation is some 10 inches or more? Answer: We need to provide for easier ionization across the greater gap. This we can accomplish by placing a candle between the conductors at the lower end. The heat of the candle creates ionization. Some say the heat of the candle "carries" the spark upward but this phrase is not correct. There is of course some thermal convection but the spark is not carried up by it.

E.- A CLASSICAL PROBLEM: A cubical frame - like the framework of a cubical box - has its edges made up of resistance wire. Let each edge of the box have a resistance R. We wish to know the total resistance between opposite vertices of the framework - that is -between the ends of the greatest diagonal. The answer is: 5/6 R. This is a classic student problem in the subject which every teacher and professor assigns some time in the subject. There are several methods of solution.

F - An aluminum slab is dropped from the hand. It falls - obviously - as a freely-falling body - with an acceleration g. We now drop it in the field of a strong Alnico magnet where the pole gap is very narrow and the field strength very high. And what do we see? It falls in a strange sluggish way as if falling in thick molasses or some such stuff. Why? As it falls it is a moving conductor in a magnetic field. So an emf is induced. This emf gives rise to a current. This current gives rise to a magnetic field. . .and so on.

So we conclude this series on Electricity and Magnetism. I hope that you have had some enchantment with the things we did and saw. All that we know comes from men and women of diverse talents and many national origins. It is appropriate that we honor them.

### Course Description

Demonstrations in Physics was an educational science series produced in Australia by ABC Television in 1969. The series was hosted by American scientist Julius Sumner Miller, who demonstrated experiments involving various disciplines in the world of physics. The series was also released in the United States under the title Science Demonstrations.

This program was a series of 45 shows (approximately 15 minutes each) on various topics in physics, organized into 3 units: Mechanics; Heat and Temperature / Toys; and Waves and Sound / Electricity and Magnetism.