210 hertz's experiments. 



place to another more quickly than the waves travel, and that is very 

 important when we want to measure the rate at which disturbances 

 travel that go as fast as light. If tlie wave travels very fast, we must 

 have a very rapid vibration, unless we have a great deal of space at 

 our disposal; for the distance between two nodes is half the distance 

 the wave travels during one vibration, and so will be very long if the 

 wave travels fast, unless the time of a vibration be very short. 

 Hence, if we wish to make experiments in this way, in a moderate 

 sized room, on a wave that travels very fast, we must have a very 

 rapid vibration to start the waves. 



II. 



In the preceding article a general method of measuring the velocity at 

 which a disturbance is propagated was described. It depended on be- 

 ing able to ])roduce a regular succession of disturbances at equalinter- 

 vals of time. These were made to measure their om'u velocity by 

 reflecting them at an obstacle. Then, by the interference of the inci- 

 dent and reflected waves, a succession of looi>s and nodes are produced 

 at intervals of half the distance a disturbance is i)ropagated during 

 the time between two disturbaiu^es. It is a general method applicable 

 to any sort of disturbance that takes time to get from one place to 

 another. It has been applied over and over again to measure the rate 

 at which various kinds of disturbance are X)ropagated in solids, liquids, 

 and gases; it was applied in a inodified form years ago, to measure the 

 length of a wave of light; and, within the last year, some of the most 

 beautiful experiments on iihotography ever described are applications 

 of this principle by Ilerr Wiener and M. Lippman. 



There are three things essential to this experiment: (1) Some method 

 of originating waves; (2) some method of reflecting them; (3) some 

 method of telling where there are loops and where there are nodes. 

 We will take them in this order: 



(1) How can we expect to originate electri(; weaves? If, when a body 

 is electrified positively, the electric force due to it exists sinuiltane- 

 ously everywhere, of course we can not expect to produce anything like 

 a wave of electric force travelling out from the body; but if, when a 

 body is suddenly electrified, the electric force takes time to reach a 

 place, we must su])pose that it is propagated in some way as a wave of 

 electric force from the body to the distant place. This of course 

 assumes that there is a medium which is in some peculiar state when 

 electric force exists in it, and that it is this peculiar state of the medium 

 which we call electric force, existing in it, that is propagated from one 

 place to another. It must be carefully borne in mind what sort of a 

 thing this is tliat we call the electric force at any place. It is not a 

 good name, — electric intensity would be a better one; but electric force 

 has come so i.iuch into use it is hardly to be expected tliat it can be 

 eradicated now. Electric force at any place is measured by the 



