THE PHENOMENA OF ELECTROSTATICS. 



195 



4 



lever 



Fig. 117. 



a fairly good insulator which does not yidd * to any great extent, 

 some sort of an electromotive-force-multiplying device is more 

 convenient and incomparably cheaper and safer than a battery of 

 dynamos or voltaic cells. 



107. The alternating-current transformer or induction coil. 



One method for multiplying electromotive force is by means of 

 the alternating - current transformer, 

 which is strictly analogous to a me- 

 chanical lever pinned to a very massive 

 movable body, instead of to a rigid 

 fulcrum, as shown in Fig. .117. Such 

 a fulcrum gives way under a steady 

 force, but it is sufficiently immovable 

 under the action of an alternating force, 

 that is, a force which is repeatedly re- 

 versed in direction. The alternating- 

 current transformer or induction coil 

 cannot be used to produce a large steady electromotive force. f 



The mechanical analogue of the alternating-current transformer. 

 A lever Aa, Fig. 1 1 7, of negligible mass, is attached to a very 

 massive body M by a pin connection. An alternating force acts 

 on the end A causing it to oscillate back and forth along the 

 dotted line with great alternating velocity /, and the end a of 



* An electromotive force of 100,000 volts connected to two metal plates each one 

 meter square with a plate of flint glass between them two centimeters thick would 

 produce about one ten-million-millionth of an ampere through the plate of glass. 

 See table of specific resistances in Art. 14. 



f Accessory devices may, however, be used in conjunction with an alternating-cur- 

 rent transformer to produce an approximately steady uni-directional electromotive force. 

 One of these devices, the mercury-arc rectifier, is strictly analogous to the mechanical 

 device called a ratchet which permits a body to move in one direction only ; and 

 another device is the commutator which is analogous to the crank which converts the 

 alternating force of a locomotive engine into a pulsating, uni-directional, draw-bar 

 pull, which may become fairly steady in value if the moving locomotive is very mas- 

 sive. It is not an exaggeration to say that no one can understand the mercury-arc 

 rectifier or the commutator or any other electrical or magnetic device or phenomenon 

 unless he can reduce it in his mind to its mechanical equivalent. See the Preface to 

 this volume. 



