Chaffee quenched spark gap
Date: 1911-1912
Inventory Number: 1997-1-0513
Classification: Spark Gap
Dimensions:30 × 35.5 × 20.5 cm (11 13/16 × 14 × 8 1/16 in.)
DescriptionBoard with wire assemblies inside a wooden box with a spark gap apparatus mounted on top of the box. White insulation is wrapped around the wires in the ring, red connect the rectangular device, and green wires connect various parts of the board.
On the left side of the board are two thick rings of wire with white fabric insulation. Coming out of these wires into the center of this circle of wire are thinner wires screwed to the board in a circle. Screwed to the center of the circle is a green fabric-insulated wire. More green wires are fastened outside the circle and connected to two white boxes and a square object with several alternating layers of iron and cork.
The two white boxes have green wires attached through holes in the bottoms of the sides. Each box is attached to the base panel by screws on either side.
The layered object in the right rear of the base panel is connected to red, green, and yellow-insulated wires. The green wires appear to be attached to the top, the red and yellow to a layer in the middle.
Each of the wire connections is attached to a knob on the other side of the board, the control panel, which has three dials. The leftmost dial, labeled "d" selects one of the connections from the white-insulated rings to their ends in the board. These connections are labeled 0 through 15 and three nonsequential labels. The first nonsequential is unlabeled, while the next two are labeled 1s, most likely. The second dial, labeled "s," selects one of the connections of the red wires running from the white rings to the board. These connections are labeled 1 through 10. The third knob has a quarter circle radiating out from it covering many of the connections, where the red wires connect the board to the multilayer apparatus. These connections are labeled 1 through 8.
The box is double hinged. The cover is hinged to the bottom piece at the rear, and the front piece is hinged to the cover. The front piece attaches to the bottom with a latch.
The spark gap apparatus is unconnected to the rest of the assembly. It consists of two roughly cylindrical configurations on either side of a square configuration that houses the actual gap. The square consists of at least two kinds of metal. The faces are the color of iron, while the sides are the color of brass or gold. The cylinderon the right of the square is a series of five discs, the air cooling vanes. The configuration is part of the left cylinder as well, but it is attached to a spool with three holes on its face.
Signedunsigned
Inscribedcontrol panel, on left dial: d
control panel, knobs surrounding left dial: 1 2 3 4 ... 15 1S 1S
control panel, on middle dial: S
control panel, knobs around middle dial: 1 2 3 ... 10
control panel, knobs surrounding right dial: 1 2 3 4 ... 8
FunctionThis device consists of a spark transmission apparatus used for wireless transmission of signals. Chaffee himself was interested in wireless telephony. The principle of the spark gap is that a capacitor would charge until the potential was high enough to break down the resistance of the spark gap. The discharge would produce an electric pulse. The full circuit would be coupled, by means of a solenoid, with another circuit containing an antenna that would radiate that pulse in the form of electromagnetic radiation. The improvement Chaffee made over other spark transmitters was developing a spark gap whose signal would instantly drop back to zero instead of persisting. This facilitated the rapid oscillation of the circuit and the transmission of high-frequency signals.
The gap itself is on top of the box and consists of an aluminum cathode and an anode, most likely of copper, inside the square case in the middle. The discs on either side of the gap are air cooling vanes to prevent overheating of the electrodes.
To understand the layout of the circuit connected to the gap, we will turn to Chaffee's 1912 paper, "A New System of Impact Excitation of Continuous Electrical Oscillations and Their Application to Wireless Telephony," Journal of the Franklin Institute 173, 5 (May 1912). Referring to a diagram on page 443 of the paper, he writes: "This circuit also contains the two inductances or choke coils L0L0, and a variable resistance R0, capable of reducing the current to about 0.2 ampere. In the following experiments L0L0 are ordinary A.C. arc-light inductances, and R0 is a resistance, adjustable in one-half ampere steps, in series with a variable copper-sulphate electrolytic resistance, for closer regulation of current. C1 and C2 are variable air condensers, G the gap, L 1 and L2 the variable primary and secondary of the closely-coupled oscillation transformer, and I2 a hot-wire ammeter. The secondary circuit is here shown as a closed circuit, but an open antenna circuit may be used."
Chaffee goes on to describe what occurs in this circuit during one cycle: "It is clearest, in tracing the sequence of phenomena, to begin at the instant after the system has been started, when the potential of the primary condenser C1 has attained a value sufficient to break down the high resistance of the gap. The gap resistance rapidly falls to a very low value as the discharge of condenser C1 and the main current Io, which was previously flowing into C1 and which remains practically constant on account of the inductances L0 L0, rush across the gap and through the primary coil L1. This current rush takes place according to a definite positive loop of current with respect to time, the shape, amplitude, and duration of which depend upon the constants of the circuit, the rapidity of absorption of energy by the secondary circuit, and, to a slight extent, upon the conditions of the gap and strength of the main current I0. The condenser C1 is discharged and charged somewhat in the opposite direction by this positive current rush. If conditions are right the discharge stops as soon as I1 becomes zero, there being no inverse current. The secondary receives a certain increment of energy from this discharge, and continues to oscillate after the primary discharge stops" (445).
Historical AttributesThe Chaffee quenched spark gap was devised by E. Leon Chaffee at Harvard University in about 1910 while working on his doctoral dissertation (completed in 1911). The Chaffee gap could produce continuous oscillations for long-distance telephone transmissions. Chaffee filed a patent for the "Apparatus for and method of exciting electric oscillations" on Novemberr 4, 1911. This is an example of one of Chaffee's experimental models.
Primary SourcesE. L. Chaffee, "A New Method of Impact Excitation of Undamped Oscillations and Their Analysis by Means of Braun Tube Oscillographs," PhD. dissertation, Harvard University, 1911.
For Chaffee's description and some good diagrams, see E. Leon Chaffee, "A New System of Impact Excitation of Continuous Electrical Oscillations and Their Application to Wireless Telephony," Journal of the Franklin Institute 173, 5 (May 1912): 437-473, available here.
ProvenanceHarvard University Department of Physics