Electrician and Mechanic reader ‘W.S.B.’ wrote in from Brooklyn in the Autumn of 1908, with the picture above, to tell about his transmitting apparatus. He said this:
“Height of aerial to pole from spark-gap, about 25 feet, and the pole is 35 feet high, with umbrella aerial radiating from top of pole (eight ribs of 7-22 copper wire), each rib about 30 feet long, or total length of, say, 90 feet. Induction coil will give a 7½ inch spark. I use six glass plates, 10 X 12, with tin foil 7 X 9 inches on each side, connected in multiple, for my sending condenser, as shown by illustration. Battery, nine storage cells, worked at about 7 or 8 amperes. Have experimented quite a lot with independent interrupters, and I have now got one that will interrupt properly at a fast rate, and under current as above, without the points welding.”
This description referred to some concepts unfamiliar to me, though I’d heard the vocabulary. I went with questions to Roger Horton, K8CIX, contemporary builder of spark stations, who learned the art from his father and uncle, who started in the radio construction business in 1919. See his magnificent site, k8cix.tripod.com/crystalradio.html. Roger very generously filled me in on technical details I didn’t understand. My questions, and his answers, are below. Anyone who wants to know how spark radio worked should read this. Here are my queries, and here is what he said.
- Were antennas in 1908 pretty improvised? His sounds like an RF mess, designed just to get as much metal into the air as possible.
Well, it was early in the science and skin effect was known, so many experimenters did just as you describe and got as much wire up as possible, disregarding frequency or resonance.
- Is the induction coil the same as a tank circuit? Is its function just to build a voltage gradient big enough to arc?
No, the Helix or Helix Coil was actually a tank circuit that helped to determine the frequency of the emitted signal. It really had no use in the ARC performance. Really just a coupling device to couple the transmitter to the aerial. The induction coil was the coil at the beginning of the circuit that actually had a high turns ratio between the primary and secondary. Supplied with pulsed D.C. these coils usually developed between 14,000 and 20,000 volts which was then supplied to the Spark Gap.
- What is the condenser for? Is it a capacitor that just smooths out current?
Adding the condenser in spark gap technology was relatively simple. It entailed adding a capacitor across the secondary winding of the induction coil (or the spark gap) used to generate the spark. The addition of this single capacitor to the spark gap transmitter made a big difference. It eliminated the continuous arc which dragged down the voltage from the induction coil. Placing the capacitor across the secondary of the induction coil, in the transmitter, enabled both the gap current and the resulting antenna current to increase, and also the fast discharge of the capacitor removed the gap resistance from the antenna circuit. Both of these attributes come as a result of the addition of the single capacitor of approximately .05uF. The time it took the capacitor to charge kept the arc from occurring and at full charge the arc would then fire, also discharging the condenser and the sequence would start over, all occurring in milliseconds. This was called the oscillation or oscillator circuit.
- What exactly is an interrupter?
There were 2 items referred to as interrupters. One, the device causing magnetic switching of the D.C. voltage supplied to the induction coil. This magnetically operated switch, called a buzzer or interrupter, usually was mounted on the front of the induction coil and the primary core of the coil caused the switch to interrupt the D.C. supply to the coil. The other type of interrupter was used in the ARC circuit to quench the spark gap and was called a Rotary Spark Gap.
- Why did the D.C. input current need to be pulsed? And I would have thought anyway that for a transformer, the current would have needed to be alternating.
Many of the places a spark gap transmitter was used had no access to A.C., but pulsed DC will operate an induction coil in place of the normal AC required to operate a transformer. Also, most of the early induction coils were designed for gas engines, such as autos, tractors, aircraft and hit & miss engines, and they ran on 6 vdc.
If you remember back, just a few years, the automotive ignition system consisted of an induction coil, a set of points (interrupters), and a condenser, to create the spark or arc for the spark plugs.
- And is the induction coil the same as a ‘loose coupler’, or is that for receiving?
Loose couplers are strictly for receiving using a crystal detector.
I am indebted to Roger Horton! This makes early radio come alive.
Text and picture: Electrician and Mechanic, October, 1908, p. 179, available at http://earlyradiohistory.us/1908wc2.htm.