Note
The National Radio Institute (NRI) was a prominent correspondence school that trained students in radio, television, and electronics from 1914 to 2002. They were well-known for their home-study courses and even created their own line of kits under the "CONAR" (COmpany NAtional Radio) brand, which allowed students to build their own test equipment, TVs, and radios.
Among their training tools, NRI developed a device called the Natrometer. The Natrometer was specifically designed for Morse code training. It was an electromechanical device, similar to other Morse code trainers like the Omnigraph, that would automatically produce the dots and dashes of telegraph code. The devices were made by the Natrometer Corporation in Washington DC exclusively for NRI, and many said “Made for National Radio Institute.”
The key difference between the Natrometer and the Omnigraph was that instead of using disks with notches representing dots and dashes, the Natrometer used a wheel with nine levels of conductive spots along the edge. This scheme is vaguely reminiscent of a printed circuit. The conductive spots were "read" by "fingers" riding along the edge of the wheel. The spots and fingers completed an electric circuit that operated a high pitched buzzer or other type of sounder. The only power required was a battery to operate the buzzer.
Each level of the wheel contained from 30 to 35 characters, so that in total, an entire wheel contained anywhere from about 270 to 315 characters.
The code wheel revolved on a spring-wound mechanism. The wheel included holes into which up to five wooden pegs could inserted. As the wheel rotated, these pegs engaged a gear that rotated a shaft with bumps that changed which of the nine fingers would be operational. This changed which level was being read and provided more than 600 combinations, effectively scrambling the messages, reducing the possibility of memorization. Pegs could be added or removed while the machine was running.
Students could set the Natrometer to send code at speeds from 3 to 40 words per minute. It was also possible to introduce a static sound to more closely resemble actual off-air traffic. The available documents do not explain exactly how this feature worked.
Users could send the wheel back to NRI and receive a new one in exchange, for only the cost their return postage. Exchanges of this kind could be made as often as the user wished.
As NRI was formed in 1914, and because the Natrometer was a key component of the company's training methodology, it follows that the device was introduced very early in NRI's history, hence the "circa 1915" date. Additionally, a document (attached) dated 1921 illustrates that the Natrometer was in use by then. As the document is a revised version, it lends credence to a date as early as 1915.
Models A and B include a key and a sounder and are the same, except that model B is provided with a wood carrying case. Model C does not have an integrated key or sounder.
