If you have researched the telegrams available at the Purdue website, you have come upon radiograms that appeared to be in some secret code, consisting of groups of five numerals, probably left you scratching your head. Well they are in code, but not a secret code. The code is the open code named the International Radio Weather Code For Use On United States Selected Ships. published in 1930. This code provides the method to compress weather reports into just a few short groups for transmission by radiogram. Why do this? Because telegrams and radiograms were expensive, you were charged by the word or by the five digit group. Because of the high cost of telegrams, many commercial code books were published so that long phrases could be reduced to one group, resulting in the savings of millions of dollars for businesses that did business by telegram. For instance, one of the most popular codes, The ABC Universal Commercial Electric Telegraphic Code, (commonly, the ABC Code) encoded the whole sentence "Will have an examination soon as possible" as one code group, 02115. And 05565 means, "Store the goods for account of whom it may concern." How much money did this save? Itasca was paying 56 cents per word or group. The ten word sentence above would cost $5.60 which is $89.60 in 2011 dollars. But replace it with its code equivalent, 05565, and it only cost $0.56, $8.96 today. Cables cost so much that it was worth it to businesses to hire full time code clerks who's only job was to encode and decode telegrams all day long using code books such as the ABC Code. This was much less expensive than paying for the full length telegrams.
The same goes for weather reports. I have attached a radiogram from the Swan and a marked up version showing the decode of the six standard code groups. I am also attaching the complete code book.
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