hertz's researches on electrical waves. 171 



coiucided, wboii tlic iniiiiinuni j)oints ajiaiii became absoluto mill poiuts. 

 These results are in agreeiuent with the conclusion drawn from the 

 former observations, for as the oscillation period of C api)roaches that 

 of A A', the intensity of the current in the former increases, but a dif- 

 ference of phase arises between it and the exciting e. m. f. When the 

 two are in unison the current in C attains its maximum, and, as in other 

 cases of resonance, the difference of phase gives rise to a slightly damped 

 oscillation, having a period of about a quarter that of the original one, 

 which makes any interference between the oscillations excited in the 

 circle BhjAA' and C respectively impossible. These conditions clearly 

 correspond to the stage at which the sparking distances at h and h' were 

 equal. When the oscillation period of C becomes decidedly greater 

 than that of A A', the amplitude of the oscillation in the former will 

 again diminish, so that the difference in phase between it and the ex- 

 citing e.m.f. will approach half of the original period. The current in 

 C will therefore always be in the same direction as that in A A', so 

 that interference between the two oscillations excited in B will again 

 become possible, and the effect of C will then be opposite to its original 

 effect. When the conductor Cwas made to approach A A' the sparks 

 in B became much smaller, which is explained by the fact that its ef- 

 fect will be to increase the oscillation period of A A', and therefore to 

 throw it out of unison with B. 



Effects of the approach of dielectrics. — A very rough estimate shows 

 that when a di-electric of large mass is brought near to the apparatus, 

 the quantities of electricity set in motion by di-electric polarization are 

 at least as large as in metallic wires or thin rods. If therefore the ac- 

 tion of the apparatus were unaffected by the approach of such masses 

 it would show that in contradiction to the theories of Faraday and 

 Maxwell, no electro-dynamic actions are called into play by means of 

 dielectric polarization, or as Maxwell calls it, electric displacement. 

 The experiments however showed an effect similar to that which would 

 be produced if the di -electric were replaced by a conductor with a very 

 small oscillation period. In the tirst experiment made, the mass of di- 

 electric consisted of a pile of books, 1.5 meter long, 0.5 meter broad, and 

 1 meter high, placed under the plates A A'. Its effect was to displace 

 the null points through about W^ towards the pile. A block of asphalt 

 (i>, in Fig. 10), weighing 800 kilograms, and measuring 1.4 meter in 

 length, 0.4 meter in breadth, and O.C meter in height, was then used in 

 place of the books, the plates being allowed to rest upon it. 



The following results were then obtained : 



(1) The spark at the highest point of the circle was now decidedly 

 stronger than that at the lowest point, which was nearer to the asphalt. 



(2) The null points w^ere displaced through about 2;3o downwards, 

 that is, in the direction of the block, and at the same time were trans- 

 formed into mere points of minimum sparking, a complete disappear- 

 ance being no longer obtainable. 



