APPENDIX 157 



others give negative electricity. "The human body, 

 when rubbed with silk, becomes negatively charged, 

 with wool, positively charged." 



Chemical electricity is the result of the action of an 

 acid on two metals, frequently zinc and carbon. That 

 which is acted on most readily, zinc in this instance, is 

 called the positive plate, indicated by the plus sign -)-, 

 while the one which resists the acid more is the negative 

 plate, indicated by the minus sign . 



Such plates, united by copper wires, which are good 

 conductors, form, when immersed in an acid solution, 

 a galvanic cell. "The ends of the wires leading from 

 the plates are called electrodes, the positive plate (+) 

 called the cathode or negative pole, while that connected 

 with the negative plate ( ) is the anode or positive pole. 

 A battery is a combination of a number of cells" (Bliss 

 and Olive). 



Some of the terms employed in electrical science are 

 the volt, or unit of electromotive force; the ampere or 

 unit of quantity which passes through a standard con- 

 ductor in a given time ; the o~hm or unit of resistance of- 

 fered by a copper wire 250 feet long and a twentieth 

 of an inch thick, and the watt, or unit of work. 



When an electric current is passed through a Crookes 

 tube it produces a ray which has the power of passing 

 through and illuminating bodies impenetrable to ordi- 

 nary light. These are called "x-rays/' or Roentgen rays, 

 and have proved of immense service to the medical pro- 

 fession in locating foreign bodies, fractures and many 

 other conditions formerly very obscure. 



MAGNETISM 



The horseshoe magnet, so familiar as a toy, is the com 

 monest and best known display of magnetic force. Any 



