268 LECTURES TO SCIENCE TEACHERS. 



in the same state. If instead of charging a battery or other 

 insulated conductor we allow the charge to pass off to the 

 earth, we get for each turn of the handle a definite number 

 of sparks, each spark corresponding to the passage of a 

 definite quantity of electricity. The quantity produced by 

 each turn of the machine is definite, but the quantity which 

 will go into the battery becomes smaller and smaller as the 

 quantity already in the battery increases. Another way of 

 measuring out the charge for a Ley den battery or any such 

 apparatus is by the employment of a unit jar, one familiar 

 form of which I have here upon the table. The quantity 

 which the second jar receives is measured by the number of 

 sparks which pass into the knob connected with the first, 

 but the point I want to draw attention to is, that the 

 sparks indicate that a definite quantity has passed into the 

 second jar. By the time the first spark comes, a certain 

 quantity has passed into this jar and by the time the second 

 spark comes the same quantity has passed in again ; so that 

 the action of the unit jar is comparable to the case of 

 lading water into a cask out of a measure of definite capacity. 

 The spark is the signal indicating that a certain definite 

 quantity has gone in. It is not that the electricity goes in 

 at the moment of a spark, but the number of sparks counts 

 the number of times which the first jar is emptied into the 

 second, so that when we want to give three units to 

 the second jar, we ought to break the connection as soon as 

 the third spark has passed, not to go on until the fourth 

 spark is nearly occurring. 



All such measurements as these I have referred to are 

 merely comparative ; if we want to know, not merely what 

 is the proportion between one quantity of electricity and 

 another, but what the actual quantity is in any particular 

 case, then we must employ a method founded upon the 

 action which the given quantity can produce, and com- 

 pare this with the amount of the same kind of action which 

 the adopted unit of electricity could produce. The most 

 obvious way of getting an absolute measure of quantities of 

 electricity is founded upon the law established by Coulomb, 

 which applies to the force exerted between two quantities of 

 electricity. If we have two equal quantities of electricity, 

 represented each by q then they attract each other if they 

 are of an opposite kind or repel each other if they are 



