506 MAGNETISM AND ELECTRICITY and today the capacity of a telegraph system has been increased still more by sending a number of messages over a single pair of wires at the same time. In actual practice eight messages are usually sent simultaneously, but more may be added if desired. Automatic typewriters, operated directly by the telegraph signals and based on the use of a group of electromagnets, form the basis of the modern teletype used in newspaper and police work. One linotype machine could operate all the other linotype machines in the United States by means of the telegraph. Lord Kelvin Perfected the Transatlantic Cable. When Lord Kelvin (1824-1907), then named William Thomson, finished his work at Cambridge, one of his examiners remarked to another, "You and I are just about fit to mend his pens." Lord Kelvin was one of those rare men, like James Watt, in whom there was a combination of mechanical ingenuity and scientific genius. There is no question as to the value of practical application as an incentive to scientific work. Some of the world's greatest scientists, like Louis Pasteur, have been spurred on to greater endeavors by the supreme desire to help solve some of the pressing problems of humanity. Lord Kelvin expressed this idea as follows: The life and soul of science is its practical application; and just as the great advances in mathematics have been made through the desire of discovering the solution of problems which were of a highly practical kind in mathematical science, so in physical science many of the greatest advances have been made in the earnest desire to turn the knowledge of the properties of matter to some purpose useful to mankind. Not only did Kelvin make contributions to the science of thermodynamics and other theoretical studies, but also did he earn the profound gratitude of all navigators and those whose lives depended on the latter for his improvements of the compass and his introduction of the sounding line. The first transatlantic submarine cable, laid in 1858, soon failed. Lord Kelvin designed a new type of strand cable better adapted to stand the strain of laying and repairing and invented the mirror galvanometer and siphon recorder for receiving the messages sent over the cables. Both instruments were applications of electromagnets, as are the majority of electrical measuring instruments. Lord Kelvin also designed electrical measuring instruments for almost every purpose. In 1896 a tremendous celebration was held in honor of this great physicist, fertile inventor, and inspiring and beloved teacher. As part
ELECTROMAGNETISM 507 of this celebration a message was sent from Glasgow to San Francisco and returned by a. different route, a total distance of 20,000 miles, in only seven minutes. What a tremendous development in communication took place in the lifetime of Kelvin ! At the time of his death there were over 225,000 miles of cables in use in the world. Electrical Measuring Instruments Employ the Electromagnet. The galvanometer consists of an electromagnet which is moved under the influence of a field of a permanent horseshoe magnet when the current passes through the coil. The coil is attached to the pointer, which moves over the scale of the instrument. Of course, the coil could be fixed in position and the permanent magnet moved, and some instruments are made that way. The relative motion of the coil and magnet is the result of the repulsion of like magnetic poles for each other. A very sensitive galvanometer is employed in the electrocardiograph to study the character of the heart-beat. It is activated by a minute electric current generated each time the heart muscles contract. Galvanometers indicate a flow of current, but they may be so designed and connected in a circuit as to measure either amperes or volts, in which case they are called ammeters or voltmeters. Some important applications of electromagnetism used in generators, motors, and transformers will be studied in the next Section. Defective Railway Rails Are Located by the Sperry Detector. The Detector Car, invented by Elmer A. Sperry, made its first commercial test on a railwoad in 1928. During the twelve years from 1928 through 1940, over 651,000 miles were tested by this device, and 345,000 defective rails were located and replaced, thus preventing an untold number of accidents in these days of higher train speeds and heavier axle loads. The Sperry Detector generates a current that passes through the rails over which it rolls. The electromagnetic field changes as the current encounters a fissure and causes a "paint gun" to squirt white paint on the defective rail. STUDY QUESTIONS 1. To whom do accidental discoveries usually come? 2. How are electromagnets used today? 3. Name two discoveries of fundamental importance in putting electricity to work. Who made these discoveries? 4. How would you construct an electromagnet? 5. How may the polarity of an electromagnet be reversed? 6. Give the principle of the first telegraph system.