ess capable transmitter invented by Bell. For the Figure 3-2 Bell's carbon microphone, still in use today Carbon Microphone Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Chapter 3 experimenters among the readers out there, it's a fun exercise to build a compressed carbon microphone. In a mortar and pestle (OK, with a plas- tic bag and a hammer) grind two charcoal briquettes to a fine powder. Pour the powder into a jar lid until the powder is even with the lip of the lid. Stretch a piece of plastic wrap tightly over the lid and secure it with a rubber band. Next, drill a hole in each side of the lid and insert an insu- lated wire that has been stripped back about a half inch to expose the copper wire. Next, connect the wires to a speaker and battery. When you speak into the "microphone," the sound waves impinging upon the stretched plastic wrap cause the carbon powder to be compressed and expanded, which in turn changes the resistance between the two con- ductors inserted into the powder. Naturally, tensions ran high between the Bell Telephone Company and Western Union during this period. In 1877, Bell offered to sell his patent to Western Union for $100,000. They turned down Bell's offer, deciding instead to buy Gray and Edison's patents and form the Ameri- can Speaking Telephone Company. Bell sued Western Union for patent infringement in 1878, and the case was settled in 1879 to everyone's satisfaction. Bell won the rights to Western Union's telephone patents, provided he pay royalties for the duration of the 17-year patent life of each device. Meanwhile, Bell Telephone Company agreed to stay out of the telegraph business and Western Union agreed to stay away from telephony. Once the legal wrangling had ended, telephone service moved along rather quickly. By the spring of 1880 the United States had 138 exchanges and 30,000 subsc