340 HOW ELECTRICITY MAY BE MEASURED 



The danger of overloading may be eliminated by inserting 

 in the circuit a fuse or other safety device. A fuse is made 

 by combining a number of metals in such a way that the 

 resulting substance has a low melting point and a high elec- 

 trical resistance. A fuse is inserted in the circuit, and the 

 instant the current increases beyond its normal amount the 

 fuse melts, breaks the circuit, and thus protects the remain- 

 ing part of the circuit from the danger of an overload. In 

 this way, a circuit designed to carry a certain current is pro- 

 tected from the danger of an accidental overload. The noise 

 made by the burning out of a fuse in a trolley car fre- 

 quently alarms passengers, but it is really a sign that the 

 system is in good working order and that there is no danger 

 of accident from too strong a current. 



313. How Current is Measured. The preceding Section has 

 shown clearly the danger of too strong a current, and the 

 necessity for limiting the current to that which the wire can 

 safely carry. There are times when it is desirable to know 

 accurately the strength of a current, not only in order to 

 guard against an overload, but also in order to determine in 

 advance the mechanical and chemical effects which will 

 be produced by the current. For example, the strength of 

 the current determines the thickness of the coating of silver 

 which forms in a given time on a spoon placed in an electro- 

 lytic bath ; if the current is weak, a thin plating is made on 

 the spoon ; if the current is strong, a thick plating is made. 

 If, therefore, the exact value of the current is known, the 

 exact amount of silver which will be deposited on the spoon 

 in a given time can be definitely calculated. 



Current-measuring instruments, or galvanometers, depend 

 for their action on the magnetic properties of current elec- 

 tricity. The principle of practically all galvanometers is as 

 follows: 



