OF DENISON UNIVERSITY. 3I 



we obtained yesterday, with the same battery. The ideal bat- 

 tery is constant in voltage and resistance whether giving a small 

 or great current. To be also efficient and convenient, it must 

 have a very low resistance. Among commercial batteries, the 

 one that most nearly fulfills these conditions is probably the Ed- 

 ison-Lalande. It delivers a large current and at a constant rate, 

 but has the disadvantage of low voltage, requiring therefore a 

 greater number of cells. It is largely used for running phono- 

 graph and fan motors. 



The secondary or storage battery has a lower internal resist- 

 ance than any primary battery, and is therefore capable of giv- 

 ing a very heavy current. It is often charged by a constant 

 primary battery of feeble power, working continuously through 

 the long intervals between short periods of use. Thus a grav- 

 ity battery of lo cells may be used to charge 3 cells of storage 

 battery. The primary battery itself would give a current of 

 only i-io to 1-5 ampere through even a low resistance, while the 

 storage battery charged by it will restore the electrical energy 

 at a rate as high as 1 5 amperes. Moreover we can predict al- 

 most with certainty just what it will do on a given circuit. It 

 must be confessed however, that storage batteries do not work 

 as well when charged in this slow way as when charged by a 

 dynamo. There is a tendency to the formation of lead sulphate 

 in the cell which interferes with its action. The result can be 

 partially prevented by introducing sodium sulphate, either di- 

 rectly or by the action of the battery acid on sodium carbonate. 

 Even without this addition, I have found this method of charg- 

 ing a few storage cells a great convenience in the physical lab- 

 oratory. 



Small dynamo machines, run by hand, are often conven- 

 ient in the laboratory, but their output is too variable and un- 

 certain to adapt them to any exact work, and the source of 

 their energy evidently unfits them for supplying current for 

 many minutes at a time. 



But electricity cannot reach the full measure of its useful- 



