3 66 THE POPULAR SCIENCE MONTHLY. 



ing. If we hang in a trough a weighted board, then, when the water 

 flows past it, the board will be pushed back ; when the current of 

 water is strong, the board will be pushed back a long way ; when the 

 current is less, it will not be pushed so far ; when the water runs the 

 other way, the board will be pushed the other way. So, by observ- 

 ing the position of the board, we can tell how strong the current of 

 water is at any time. Now, suppose we wish to know, not how strong 

 the current of water is at this time or at that, but how much water 

 altogether has passed through the trough during any time, as, for in- 

 stance, one hour. Then, if we have no better instrument than the 

 weighted board, it will be necessary to observe its position continu- 

 ously, to keep an exact record of the corresponding rates at which the 

 water is passing, every minute, or better every second, and to add up 

 all the values obtained. This would, of course, be a very troublesome 

 process. There is another kind of instrument which may be used to 

 measure the flow of the water : a paddle-wheel or screw. When the 

 water is flowing rapidly, the wheel will turn rapidly ; when slowly, the 

 wheel will turn slowly ; and, when the water flows the other way, the 

 wheel will turn the other way, so that, if we observe how fast the 

 wheel is turning, we can tell how fast the water is flowing. If, now, 

 we wish to know how much water altogether has passed through the 

 trough, the number of turns of the wheel, which may be shown by a 

 counter, will at once tell us. There are, therefore, in the case of water, 

 two kinds of instruments, one which measures at a time, and the other 

 during a time. The term meter should be confined to instruments of 

 the second class only. 



As with water so with electricity, there are two kinds of measuring 

 instruments : one, of which the galvanometer may be taken as a type, 

 which shows by the position of a magnet how strong a current of 

 electricity is at a time ; and the other, which shows how much electric- 

 ity has passed during any time. Of the first, which are well under- 

 stood, I shall say nothing ; the second, the new electric meters and the 

 corresponding meters for power, are what I have to speak of to-night. 



It is hardly necessary for me to mention the object of making 

 electric meters. Every one who has had to pay his gas bill once a 

 quarter probably quite appreciates what the electric meters are going 

 to do, and why they are at the present time attracting so much atten- 

 tion. So soon as you have electricity laid on in your houses, as gas 

 and water are laid on now, so soon will a meter of some sort be neces- 

 sary in order that the companies which supply the electricity may be 

 able to make out their quarterly bills, and refer complaining customers 

 to the faithful indications of their extravagance in the mysterious cup- 

 board in which the meter is placed. 



The urgent necessity for a good meter has called such a host of 

 inventors into the field that a complete account of their labors is more 

 than any one could hope to give in an hour. Since I am one of this 



