THE CONQUEST OF NATURE 



lightning may be considerable, as everyone is aware. 

 But, on the other hand, while the visible effect of a stroke 

 of lightning on a tree trunk, for example, makes it 

 seem a powerful agency, yet the actual capacity to do 

 work the power to move considerable masses of mat- 

 ter is extremely limited. The effect on a tree trunk, 

 it will be recalled, usually consists of nothing more than 

 the stripping off of a channel of bark. In other words, 

 the working energy contained in a seemingly powerful 

 supply of statical electricity commonly plays but an 

 insignificant part. 



The working agent, and therefore the form of elec- 

 tricity which concerns us in the present connection, is 

 the dynamical current. This may be generated in 

 various ways, but in practice these are chiefly reducible 

 to two. One of these depends upon chemical action, 

 the other upon the inter-relations of mechanical mo- 

 tion and magnetic lines of force. A common illustra- 

 tion of the former is supplied by the familiar voltaic 

 or galvanic battery. The electromagnetic form has been 

 rendered even more familiar in recent times by the 

 dynamo. This newest and most powerful of workers 

 will claim our attention in detail in the succeeding 

 chapter. Our present consideration will be directed 

 to the older method of generating the electric current 

 as represented by the voltaic cell. 



THE WORK OF THE DYNAMICAL CURRENT 



Let us draw our illustration from a familiar source. 

 Even should your household otherwise lack electrical 



