翻译下列句子: 1. No such limitation is placed on an alternating-current machine; here the only requirement is relative motion, and since a stationary armature and a rotating field system have numerous advantages, this arrangement is standard practical for all synchronous machines rated above a few kilovolt-amperes. 2. For large motors, therefore, a phase-wound rotor is used, with the winding connected at one end to each other and at the other end to slip-rings, thus enabling the resistance of the rotor to be varied at will, providing a greater starting torque and some speed control. 3. The resistance of any length of a conducting wire is easily measured by finding the potential difference in volts between its ends when a known current is flowing. 4. A diode placed in a circuit acts like a valve, allowing current to flow in only one direction. 5．A few years later he was able to take out his first two patents for high-frequency AC generators and transformers. 6. These are the field winding, which produces the flux density, and the armature winding, in which the “working” emf is induced. 7．Pure science has been subdivided into the physical science, which deals with the facts and relations of the physical world, and the biological sciences, which investigate the history and working of life on this planet. 本文来源：https://www.wddqw.com/doc/ef48b09d80eb6294dd886cac.html