ELECTRICITY. 77 



poses, sets of coils, known as resistance coils, having a known resist- 

 ance, are used for measuring electrical resistance. 



An ampere is the unit of quantity of current, which may be deter- 

 mined by measuring the amount of oxygen and hydrogen liberated 

 by the current from water within a given time. 



The weight of copper deposited electrolytically by the current is used also 

 as a means of determining its amperage. One ampere of current deposits 1.177 

 grammes of copper per hour. For practical purposes, instruments known as 

 amperemeters, or ammeters, are used for measuring amperage; use is made in 

 these instruments of the fact that a current deflects a magnetic needle, which 

 is made to move over a dial-face graduated in amperes. 



In using electric currents for medical purposes, 1 ampere is often too strong 

 for the tissues of the body. For this reason the unit is divided into 1000 parts, 

 each part being designated as a milliampere. 



The volt is the unit for electromotive force, and is the pressure 

 required to maintain a current of 1 ampere through a resistance of 

 1 ohm. 



The relation existing between these three units is expressed in 

 Ohm's law : The strength of the current is equal to the E. M. F. 

 divided by the resistance of the current. 



The chief diiference between frictional and galvanic electricity is 

 that the first is of high tension but small in amount, while current 

 electricity is of low tension but greater in amount. 



Electromagnets. When a piece of insulated wire is wound in 

 spiral form around a bar of soft iron, and an electric current is made 

 to pass through the wire, the iron becomes a magnet for the time 

 being, and is then called an electromagnet. If a piece of steel is 

 treated in the same way, it remains a magnet even after the current 

 is broken, or after it has been taken from the spiral. 



As the strength of an electromagnet is proportional to the strength 

 of the current and the length of wire wound around it, electromagnets 

 of very much greater power than ordinary magnets can be made 

 The position of the north and south poles of the magnet depends on 

 the direction of the current, a reversion of the latter producing a 

 reversion of the polarity in the electromagnet. Practical use is made 

 of the electromagnet in telegraph instruments, in the telephone, the 

 electric bell, and in many other contrivances. 



Electricity generated by magnetism. Not only can magnets be 

 made by subjecting iron to the influence of electric currents, butj 



