The operator, in sending signals, opens the circuit, the magnets cease to hold down the armatures, and they are raised by springs and strike against metallic stops above. It is customary to say that the circuit is completed through the earth. This statement misleads some persons into imagining an electric current capable of corroding water pipes and decomposing chemical compounds, passing through the earth between stations. [Illustration: Photograph by Helen W. Cooke. Testing the Telegraphy Outfit] Perhaps it will help to a better understanding of the truth if we think of a city pumping water out of the ocean, say to fight fire, and disposing of it again into the ocean. The ocean currents thus produced are not likely to be destructive. Indeed, just as we measure height from the ocean level as zero, so we measure electric pressures as from the zero level of the earth's electrical state. [Illustration: Fig. 31] The key used by telegraphers is represented in Fig. 31. It has connected with it a switch to keep the circuit closed when the key is not in operation. The Morse code of signals consists of dots and dashes, when printed, as follows: a . - b - . . . c . . . etc. Operators learn to read the message by the intervals between sounds. A dot consists of two taps of the sounder with a short interval between, and a dash consists of two taps with a longer interval between. One tap of the sounder is caused by its descending upon the metal stop below and another by its rising against the upper stop. Telegraph sounders are operated on about a quarter of an ampere of current if from a battery circuit, or on about one tenth of an ampere from a dynamo circuit. The dynamo circuit is supplied with more volts of electric pressure, and hence its power is ample to cause the armature to strike the metal stops hard enough to be heard by the operator. For example a battery circuit may supply to the sounder a current with these characteristics: 2 volts × .25 amperes = .5 watts, while a dynamo circuit may give: 6 volts × .1 ampere = .6 watts. Telegraph line wires are usually bare, the insulation being merely the glass knobs at the poles. Clean water is a very good insulator but dirty water is a fairly good conductor. A wet telegraph pole may bring so much current to earth as to prevent all sounders on the line from operating. Hence the line is separated from the poles by glass. The poles are about one hundred and thirty-two feet apart, making forty to the mile. The wires are usually galvanized iron one sixth of an inch in diameter. Copper conducts six times as well as iron, and is now replacing iron in the lines.