Here is a great introduction to a practical application of electromagnetic theory—the field telephone. It’s a training film from 1961 that covers the sound-powered, local battery, and common battery systems along with the six basic components they use: generators, ringers, transmitters, receivers, induction coils, and capacitors.
Clear illustrations and smart narration are the hallmarks of these Army training films, and this one begins with a great explanation of generator theory. The phone’s ringer uses electromagnetic attraction and repulsion to do the mechanical work of striking the bells. Similarly, the sound waves generated by a caller’s speech move an armature to create an alternating electrical current that is transmitted and converted back to sound waves on the receiving end.
In the local battery system, the battery pushes pulsating DC to carry the voice transmission. An induction coil increases the capabilities of this system, but capacitors are required to filter out the frequencies that would overload the receiver, passing only the higher speech frequencies.
In order for several stations to communicate, the use of a switchboard is required to patch the calls through. There are many advantages of a common battery system with regard to call switching: no local battery is necessary, nor is a generator needed at each station. Calls are easier to place, and communication is much faster.
Part One:
http://www.youtube.com/watch?v=BKfa27PTE_s
Part Two:
http://www.youtube.com/watch?v=jKvllL2zxGo
Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.
Hey they didn’t mention about hooking up the generator to an enemy prisoner’s genitals, is there a third part to the film?
kek
Meh… We used to send the new privates out with the comm wire and tell them to hold the ends tightly to be sure they didn’t short out. Once they were a ways out, we’d crank the generator and watch ’em jump. Ah the fun gags you get to pull on noobs.
We actually deployed AND used these in our tent hospital during training this summer, when the voip and vsat phones crashed. Old bastards, but still work!
Awesome!
The old ones can’t NOT work! It’s just not built into em! No need for an operating system, drivers, virtual memory, or even the ability to add 2 numbers together. They work as phones because that’s what they ARE, not just their assigned task. Doesn’t matter how many Gigaflops your laptop has either, If you need planks nailing together a hammer does it better.
Except maybe the old IBM Thinkpads, they built them solid.
You can “Tell a phony in the field” but he still won’t listen to you. (groan!)