100 PRACTICAL LESSONS IN SCIENCE. 



are converted into electric pulses in the transmitter, and these 

 electric pulses reaching the receiver are converted back again 

 into sound waves. 



The Telautograph is an electrical instrument by which writing 

 or drawing may be reproduced at a distant station. "A common 

 lead pencil is used to write the message; near its point are fast- 

 ened at right angles to each other, two silk cords, which, con- 

 necting with the instrument, follow the motion of the pencil and 

 control the receiving pencil at the other end." 



"At the receiving station two aluminium arms hold the ca- 

 pillary glass tube which serves as a pen. This pen is guided by 

 the electrical impulse from the sender, and moves simultaneously 

 and in like direction and extent with every motion of the distant 

 pencil, so that the ink tracing which results must be a fac-simile 

 of whatever the sender writes or draws." It conveys a written 

 message, as the telephone conveys a spoken message. 



A piece of No. 30 copper wire about 9.7 feet long has a resist- 

 ance of one ohm, and is used as a unit in measuring the resistance 

 of wires to the passage of electricity. The difference in potentials 

 is measured in volts, and the unit of current strength is the 

 ampere, but these units are not as definite as the ohm. Many 

 other interesting subjects might be discussed under the head of 

 physics, but the subjects considered seem to be sufficient for the 

 development of the general principles of this branch of science 

 and for the illustration of their more important applications. 



