I acquired this antique electronic device a number of years ago and it was a mystery as to what it is. Opening it up I eventually figured out its purpose.

Supplies

Step 1: Examining the Device From the Outside.

The device is a well constructed wooden box with a coil form on the top that has two coils on it. There is a miniature Edison screw in style socket for a light bulb and a standard size telephone jack. There are two 1920's style binding posts into which there are two metal probes held in place. The probes are attached to two 1920's style cloth covered wires (2 feet long) attached to a small coil that looks like a wand. On the front of the box is a knob with a pointer that points to divisions marked in Braile numbers.

Step 2: Taking the Box Apart

The box consists of two halves with the top half which has the binding posts, headphone jack and coil socket on it. There are a few wood screws on the side of the bottom half that need to be taken out in order to take the two pieces apart. The top piece with the binding posts and coil for actually has all the components attacked to it so it makes it easier to take the two halves apart.

Step 3: Examining the Inside

The inside of the device consists of the bottom of the coil socket and binding posts, a variable capacitor with a fixed portion and a trimmer portion, two resistors a bulb socket and a fixed, probably mica, capacitor.

Step 4: Examining the Circuit, I Drew a Schematic

The "wand" is a small coil of wire that is to be held near the tank circuit of a transmitter. The circulating currents in the tank circuit of the transmitter are coupled into the resonant circuit formed by the coil on the top of the box and the variable capacitor, through the wires connecting the wand to the binding posts which are connected to the coupling coil also on the coil form on the top of the box. When the resonant circuit in the box is tuned to the same frequency as that of the transmitter, it can be determined what the frequency is from the markings on the front of the box. They are in braille for convenience of a blind amateur operator. Across the resonant circuit is a series circuit which consists of a 1.5 megohm resistor in series with a .2uF capacitor in parallel with a bulb socket. These are in series with a 3.6 megohm resistor which is in series with a "BH tube". I had never heard of one of these tubes and apparently it's a cold cathode rectifier. The other side of the tube is connected as a half-wave rectifier to the other side of the resonant circuit. When the resonant circuit is resonant with the one in the transmitter, voltage multiplication occurs, to produce enough voltage to ionize the tube (80 volts). The tube has some helium in it so it only allows the tube to convert a small portion of the circulating currents into a pulsating DC that will produce a sound in the headphone. The circuit will work withouut the bulb in it but will not give an indication without the headphone. I believe it was designed to work with both headphones and bulbs in it, but if the bulb burnt out, the headphone would still give the indication of resonance. It's a brilliantly designed device considering it's age and the primitive components of the time. I wanted to determine what the frequency range of the resonant circuit was so I used a modern antenna analyzer for this purpose, the lower frequency is 2775 kHz and the upper frequency is 3000 kHz. This doesn't correspond with any modern ham band but it's possible that they used that frequency range back in those days for hams. The part of the variable capacitor that isn't connected to the rotor can be moved with a bit of effort by using a screwdriver. It looks like it could be moved up to the modern 80 meter ham band around 3500 kHz.

Step 5: Dating This Device

I did some research on the BH tube, and apparently, it was designed in 1925, until 1927, the tube had a bump on the top where the air was evacuated from it and the helium was pumped in. After 1927, the tube had a smooth top as the air was them evacuated from the bottom before the socket was put on. This particular tube is from after 1927, so I would date this tuning indicator as being from 1927 or after. Anything up to 97 years old.

Step 6: An Interesting Bit of Information About the BH Tube.

Doing some research on the BH tube revealed some interesting information. I was the first profitable device that Raytheon came out with and they marketed it through the National Carbon Company (Eveready Battery). My tube here says both Raytheon and National Carbon Company on a sticker on the tube. The tube was used extensively in battery chargers and battery eliminators for battery radios in the 1920's and 30's. I don't know why these weren't used inside radios as opposed to hot cathode rectifiers. There must've been some technical reason or they would've used them to save on power.