ELECTRICAL MACHINERY 471 



become very much weaker, but if the copper plate be 

 removed and wiped, then reinserted, the current will be 

 as strong as ever. From this it is learned that the hydro- 

 gen bubbles collect on the copper and form an insulator, 

 so that the chemical action is retarded. This forming of 

 hydrogen bubbles is known as polarization, and in a good 

 cell there must be some means to check it. 



The various forms of cells now in use differ from the 

 above only by using different electrodes and having some 

 method for checking polarization.* 



626. Dry cells. Dry cells differ from liquid cells only 

 in that the exciting fluid is formed into a jelly or held in 

 suspension by some absorbent such as sawdust or pith. 



In the common commercial type the zinc element is 

 in the form of a cylinder and holds the exciting fluid and 

 carbon. The ends of the cylinder are generally sealed with 

 wax. The following proportions by weight will make a 

 very good cell : I part zinc oxide ; i part sal ammoniac ; 

 3 parts plaster ; i part zinc chloride ; 2 parts water. 



627. Heating effect of an electric current. Owing to 

 the resistance to an electric current passing through a 

 conductor, heat is developed. If the current is small and 

 the cross-section of the conductor large the amount of 

 heat developed will hardly be noticeable, but if the cur- 

 rent is strong and the conductor small in cross-section, 

 the latter will soon become hot, often red hot, and some- 

 times melt down. It is due to this heating effect that 

 many machines are burned out, and it is also due to this 

 same effect that more machines are saved. 



628. Fuse. If a piece of copper wire is connected in 

 series with one of lead and a current sent through them 

 the lead will melt down at a little over 600 F., but it 



*For discussion of commercial cells see any text book on physics 

 or elementary electricity. 



