hertz's experiments. 207 



actions were not propa.uatod instiuitiineously, bnt were propagated 

 witli the velocity ol" light. Accordiuu' to his theory an electric distnrb- 

 ance occurring on the sun wonhl not ])i'odnce any <'i'fect on the earth 

 tor about eiglit minutes after its occurrence on the sun. No exi)eri- 

 nients were known to test the trutli of this deduction until the genius 

 of Jlertz brought some of the most beautifully conceived, ingeniously 

 devised, and hiborinusly executed of ex])eri7nents to ;>. brilliantly suc- 

 cessful conclusion, and demoiistrutcd the juopagation of electric and 

 magnetic actions with, the velocity of light, and thereby i)rove<l experi- 

 mentally that they are due to that sanu' wonderful, all-pervading 

 medium by means of which wc get all the energy that makes life here 

 possible. 



The i)roblem to be solved was, arc e]ectri<' and nmgnetic actions propa- 

 gated from place to place in a Unite time, or are they simultaneous 

 everywhere? How can exiieriments be made to decide this? Consider 

 the corresponding problem in sound. What methods are there for de- 

 termining the rate at which sound is i)ropagated? An experiment that 

 measures the rate can tell whether that rate is tiuite or whether it is 

 infinitelj^ great. There are two important methods employed for meas- 

 uring the velocity of sound. The second is really only a moditication 

 of the first direct method, as will be seen. The direct method is to nmke 

 a sudden sound at a ])lac{i and to find how long afterward it reaches 

 a distant jdacc. In this juethod there is required some practically in- 

 stantaneous way of comnmnicating between the two i)laces, so that the 

 distant observer may know when the sound started on its journey. A 

 modification of the method does not require this. It depends on tiie 

 use of refiection. If a sound be made at a distance from a refie(;ting 

 surface, the interval of tinu' between when the sudden sound is made and 

 when the retlectcd sound (the echo) retui'iis, is the time the sound took 

 to travel to the retlcctor ami back again. A well-known modification 

 of this method can be apjdied if we can secure a succession of smldcn 

 sounds, such as taps, at accurately equal intervals of time, We oiigi- 

 nate such a regular succession of taps, and alter the distance from llie 

 rertector until each refiected tai)Occuis sinuiltaneously with the succeed- 

 ing incident ta]>. Or if the distance at which we can i)ut the refiector 

 be sufficiently great, wc may arrange it to l)e such tint a icfiected tap 

 is heard sinuUtancously Avith the second, third, fouilh, or any desired 

 succeeding ta]). The coincidence of the taps with their refiections can 

 be fairly accurately observed, ami a fairly accurate estimate foi med of 

 the velocity of sound, /, c, the velocity at which a con>pressingor rare- 

 fying of the air is i)roi)agated by the air. Instead of altering the dis- 

 tance of the source of sound from the reflector, we may ourselves move 

 about between the souiceand the refiector, and we can find some places 

 where the reflected taps occur simultaneously with the incident taps, 

 and some ])laces where they occur between the incident on(\s. This is 

 pretty evident, for if we start from the source toward the reflector, as 



