Electronic Brain
January 1962 Electronics Illustrated

Back in the 1960s, Electronics Illustrated magazine ran a series of monthly Q&A columns titled "Electronic Brain," where readers wrote in to query the staff on particular quandaries. Even if you have been in the electronics game for decades, there were plenty of questions that probably invoked the "I'm sure I could have answered that at some point, but it's been so long that I couldn't say for sure," thought. The magnetomotive force topic in this set of three items did it for me. I knew there was a magnetic flux equivalent of electric current flow, but I probably would not have been able to write the equation using the precise technical terms. The other two were simple enough. How did you do on these? 

Electronic Brain

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Mercury in Space

Will a mercury switch operate in space where there is no "up" or "down ?"
- Martin Alter Bridgeport, Connecticut

A mercury switch contains a globule of this liquid metal in an evacuated glass tube. When operated, the mercury falls by gravity to a lower point in the tube  where it establishes electrical connection between two electrodes and completes a circuit. Under conditions where a man can float around inside a space ship, so would the mercury globule; and its ability to act as a switch would vanish.

Lamp Brightness

While experimenting with ordinary incandescent lamps, I . found to my amazement that when a 50 -watt lamp is connected in series with a 100-watt lamp the 50-watt lamp glows more brightly. This is the opposite of what happens when these lamps are used in house fixtures. What causes this?

- Peter Neumann Olkegger, Kentucky

The brightness of a lamp depends entirely upon the amount of power being dissipated in its filament. In a series circuit, the current in every component is the same as in every other component. This means that the factor that governs the brightness of the lamp must be its resistance in a series circuit. This comes from the equation:

W = I²R

in which W = power in watts, I = current in amperes, and R = resistance in ohms. Since the I is the same for both lamps, only the R can affect the power.

The nominal resistance of an operating 100-watt lamp on a 120 volt house line is 144 ohms. The resistance of a 50-watt lamp under the same conditions is twice this value, or 288 ohms. Since the 50-watt lamp has the higher resistance, it will dissipate more power when connected in series with the 100-watt lamp and it therefore glows brighter.

Gilbert's Force

I recently ran across a reference to something called magnetomotive force measured in gilberts. Can you explain these terms?

- Gene Clough Long Beach, California

In an electric circuit, the difference of potential between the terminals of a battery is called the electromotive force because it causes the current to move through the resistance. In magnetism, a similar effect exists: when a current flows through a coil of wire it sets up a magnetomotive force which causes a magnetic flux (lines of force) to move around through the reluctance of the magnetic circuit. Reluctance is determined by the type of core material, its cross -section, and its length. Thus, it bears some resemblance to electrical resistance.

A law very similar to Ohm's Law governs magnetic circuits. For electricity Ohm's Law reads:

current = EMF Resistance

and for magnetism, the law reads:

MMF flux - Reluctance

where MMF is magnetomotive force.

Magnetomotive force is most often measured in terms of ampere-turns. Thus, a coil of 100 turns carrying 2 amperes has a magnetomotive force of 200 ampere -turns. Another unit of magnetomotive force sometimes used is the gilbert. The gilbert is defined as 47r/10 ampere-turns or 0.794 ampere-turn. The gilbert is, therefore, a somewhat smaller unit of magnetomotive force than the ampere-turn.