266 LECTURES TO SCIENCE TEACHERS. 



quantity of electricity which we have to measure can produce, 

 and compare this with the magnitude of the similar effect 

 produced by the standard quantity. This rather abstract 

 way of stating the case will be more intelligible if I give you 

 an example. In the case, for instance, of heat we are pretty 

 much in the same circumstances as in relation to the measure- 

 ment of electricity. We cannot take a unit of heat like a 

 unit of length and compare it with other quantities. We 

 have to produce some effect by the quantity of heat we want 

 to measure. We may for instance employ it to raise the 

 temperature of water from the melting-point of ice to one 

 degree and estimate the quantity of heat by the amount of 

 water whose temperature can so be raised from zero to 1 C, or 

 we may use the heat in melting ice and estimate the quantity 

 of heat by the quantity of ice that it can melt, the unit of heat 

 being of course either that quantity which can raise a unit 

 mass of water one degree, or that which would melt a unit 

 mass of ice. 



So in the case of electricity, we may adopt as a standard 

 quantity, as the unit of electricity, the quantity which can 

 produce some definite effect, and then to measure other 

 quantities we find out how much of the same effect they can 

 produce. If we want to get what we may term an absolute 

 measure some such process as this is always necessary, but 

 where what we require is simply to compare one quantity 

 with another without requiring to know absolutely what 

 either of them is if we want to know for instance that one 

 quantity is 100 times another without knowing how much 

 this one is for mere comparisons of that sort, many other 

 processes are available : for instance, we may adopt, as 

 giving a comparative measure of two quantities of electricity, 

 the number of times which some definite operation has to be 

 repeated in order to produce the one quantity or the other. 

 The number of turns of the handle of a given machine for 

 instance, might be taken as a measure of the quantity of 

 electricity which is employed in a particular experiment. If 

 we turn the handle 10 times we produce a definite quantity : 

 if we turn it 20 times we produce twice as much, when the 

 machine is in the same condition ; but as all of you know who 

 have worked with electrical apparatus, one day a machine 

 will give us much larger quantities than it will another day, 

 when employed in the same manner, so that such a method 



