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    "That is certainly one type of interference," said Miranda, "and that is why my father moved to this house in the country, somewhere where the interference is lower. However a lot of interference comes from thunderstorms, and at these low frequencies, thunderstorms anywhere in the world cause interference." 
    "Did he make a different antenna ?" asked Bruce. 
    "Yes he tried several antennas," said Miranda, "and finally he made the big rod antenna you can see at the end of the garden."
    "I always wondered what that was," said Bruce. "It must be 3 metres long, and it looks as if your father can turn it to point in any direction. Why did you call it a rod antenna ?" 
    "Well," said Miranda, "it works in the same way as the rod antenna in a portable radio, but it is made with hundreds of wires of a special steel called mu-metal. He had to paint each wire, before he tied them all together, to make sure they did not touch each other. Then he wound a coil of wire round it all, using many turns of wire. It took a long time to make. The antenna is on the pole, so that he can turn it to face in any direction." 
    "Ah," said Bruce, "was that to find where a signal was coming from ? I can turn my portable radio round slowly and find a position where the signal suddenly disappears, and this shows me the direction of the radio station, like a direction finder on a small boat." 
    "Yes," said Miranda, "that's right." 
    "What signals did he receive ?" asked Bruce.
    "Well he spent a long time listening to many different frequencies," said Miranda. "Actually, he does not listen, but he prints the signals on a roll of paper. This is because the signals change very slowly, and he looks for a pattern. At most frequencies the signals look like a meaningless squiggle on the paper, but near 12 hertz he saw a very weak signal with a definite pattern which repeated every 40 minutes." 
    "How exciting," said Bruce. "What was the pattern ?"
    "The pattern is like dots of Morse Code," said Miranda. "First there was one dot, then 2, then 3 and the whole pattern is 1,2,3,5,7,11,13,17. Let me write them down for you. Do you notice anything unusual about the numbers ?"