318 Prof. Helmholtz on the Methods of Measuring 



spot, in the rotating mirror they will not appear in the same 

 place if, in the interval between them, the position of the mirror 

 has become sensibly changed; by measuring the angle under 

 which the images appear, the interval between them may be 

 calculated. The invention of the rotating mirror is due to 

 Wheatstone, who made an experiment with it to determine the 

 velocity of propagation of the discharge of a Leyden battery. 

 The most striking application of the idea was made by Fizeau 

 and Foucault during the present year, in carrying out a propo- 

 sition made by Arago soon after the invention of the mirror ; 

 we have here determined in a distance of 12 feet no less than 

 the velocity with which light is propagated, this is known to be 

 nearly 200,000 miles a second ; the distance mentioned corre- 

 sponds therefore to the 77 millionth part of a second. The 

 object of these measurements was to compare the velocity of light 

 in air with its velocity in water, which, when the length is 

 greater, is not sufficiently transparent. The most complete 

 optical and mechanical aids are here necessary ; the mirror of 

 Foucault made from 600 to 800 revolutions in a second, while 

 that of Fizeau performed 1200 to 1500 in the same time. 



Finally, I have to mention a method of measuring time which 

 depends upon a totally different principle. I have already indi- 

 cated it by saying, that the time is to be calculated from the 

 effect which a force of known magnitude is able to produce 

 during the time. This force is the electro-magnetic action of a 

 spiral of copper wire upon a magnet suspended by a fibre, Imerely 

 remind my hearers that a spiral composed of covered copper wire 

 acts as a magnet, having a south pole at one end and a north 

 pole at the other, as long as a voltaic current circulates through 

 it. In the neighbourhood of this spiral let a magnet be freely 

 suspended. As long as no current is present, the magnet per- 

 forms smaller or larger oscillations under the directing influence 

 of the earth's magnetism, which diminish with extreme slowness 

 and never entirely cease, inasmuch as feeble currents of air and 

 alterations of the earth's magnetic force constitute ever-new 

 sources of motion. Let a current pass through the spiral. As 

 long as it continues, one pole of the magnet is attracted by the 

 adjacent end of the spiral and the other pole repelled. The 

 motion of the pole will be thus changed ; and according as its 

 direction coincides with, or is opposed to that of the electro- 

 magnetic force, it will be accelerated or retarded, or perhaps 

 reversed. As soon as the current has ceased, the magnet once 

 more makes regular oscillations, the magnitude of which changes 

 very slowly, and hence can be determined with ease. These 

 oscillations, however, on account of the motion imparted by the 

 voltaic current to* the magnet, will not be of the same magnitude 



