RESISTANCE DEVICES 



151 



rary arrangements of apparatus. Fine wires may be corroded off 

 after a short time. Provision must be made for safely replacing 

 the water lost by boiling. 



TABLE OF APPROXIMATE DATA CONCERNING CARRYING CAPACITY OF 

 GALVANIZED-IRON WIRE WHEN IMMERSED IN WATER 



The heating of these wires when immersed is so great that 

 there must be no obstruction to a free circulation of the cooling 

 water. Strong strings or fairly sharp edges of wooden sticks 

 will make reliable supports. The water used must be clean, 

 to prevent rapid destruction of the wires by electrolysis. 



Drop Wires. A very useful form of drop wire for controlling 

 the voltages applied to the potential coils of instruments may 

 be made by winding a single layer of double cotton-covered 

 resistance wire on a piece of brass tube about a meter long and 5 

 cm. in diameter, which has been slit lengthwise and covered with 

 stout paper. The insulation is sandpapered off where the slider 

 makes contact. By use of this device, the voltage in a derived 

 circuit may be adjusted from zero to a maximum. It should 

 not be forgotten that the arrangement is a long solenoid and will 

 have a considerable stray field. 



Rheostats similar to those shown in Fig. 79 are regularly on 

 the market, and are very convenient for general laboratory pur- 



