20 has four rotor arms, four brushes, and four commutator contacts. Some high-performance DC motors do not use wound rotors, but instead print the rotor winding as traces on a printed circuit. This provides a very low- inertia motor, capable of high acceleration. DC motors do not lose synchronization like stepper motors do. If the load increases, the motor speed decreases until the motor eventually stalls and stops turning. DC motors are typically used in embedded systems with posi- tion encoders that tell the microprocessor what the motor position is. Encoders will be covered in detail later in the chapter. A DC motor is typically driven with an H-bridge, like a bipolar stepper. However, a DC motor requires only one bridge circuit, because there are only two connections to the motor windings. DC motors will typically operate at higher speeds than equivalent stepper motors. Driving DC Motors Like steppers, a DC motor can be driven with an on-off chopped H-bridge or
an ambassador's car from the Foreign Office building. With this little datum, the cryptanalysts—knowing the message he had been given and estimating how long it would take the ambassador to drive to the embassy and have a message of that length encoded and sent to the telegraph office—could more easily pick out the cryptogram corresponding to that message from the embassy's daily file. As usual, the Swedish cryptanalysts were greatly helped by lazy or stupid encoders. Clerks repeatedly violated the most elementary rules by failing to superencipher and forgetting to bisect messages. The worst bungler the Swedes came across was the German consul at Stavanger, whose numerous blunders became the vulnerable heel of many a German message. His name—almost too fittingly—was F. W. Achilles. The Swedes appreciated his help so much that they hung a large photograph of him in their office. "He was very fat and he looked like a gorilla," Segerdahl said
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