MAGNETIC EFFECTS OF ELECTRIC CURRENTS 233 



the pointer showing about 1.5 volts. The direction of rotation 

 can be reversed by exchanging the wires on the terminals of 

 the cell. 



The movable coil carries a small current and swings about 

 an iron core. The coil therefore has all the properties of a 

 magnet, with a north pole on one side and a south pole on 

 the other. The motion, then, is due to the action between 

 the magnetic poles of the coil and the poles of the permanent 

 magnet. A spring is used to bring the coil to its original posi- 

 tion. Many common electrical devices for measuring electric 

 currents are merely modifications of the simple motor. 



238. Danger of electrical injuries. If precaution is taken 

 not to connect the dry cell directly to the 80-millivolt scale, 

 which would burn out the coil, an additional experiment of in- 

 terest may be shown. The human body has a very high resist- 

 ance, from 1000 to 10,000 ohms or more, depending largely 

 upon the condition of the skin. According to Ohm's law 

 (sect. 218) a dry cell (1.5 volts) ought to force about one 

 thousandth of an ampere through a resistance of 1500 ohms. 

 The circuit of the millivolt meter is designed to carry only 

 a few hundredths of an ampere, and the pointer will register 

 the small current which flows through the body when the 

 hands grasp the terminals at the bottom of the voltmeter. 

 If the fingers are wet, thus reducing the resistance, the 

 pointer indicates that more current passes. A still greater 

 current is forced through the body when the hands are wet 

 with salt water. A current of about ten amperes is used in the 

 electrocution of criminals. As low an amount as one-tenth 

 ampere may be dangerous under certain conditions. Work- 

 men regard from 400 to 600 volts as sufficiently high pressure 

 to force enough current through the body to be fatal. Cases 

 are recorded in which the 110-volt lighting circuit proved 

 dangerous. Many small motors operate upon 110- and 220- 

 volt circuits. Street-car and subway motors usually use a 

 600-volt circuit. 



