72 Professor Oliver Lodge [Feb. 24, 



WEEKLY EVENING MEETING, 



Friday, February 24, 1899. 



Sir William Crookes, F.K.S., Vice-President, in the Chair. 



Professor Oliver Lodge, D.Sc. LL.D. F.E.S. 



Coherers. 



A coherer is an instrument which responds to electric waves 

 somewhat in the same manner as a microphone responds to sound 

 waves. 



A coherer is a light metallic contact or series of contacts 

 introduced into an electric circuit of low voltage containing also a 

 galvanometer or other signalling instrument. A steady current is 

 normally unable to pass, or only very feebly, by reason of the high 

 resistance of the bad joint, but under the influence of a sudden change 

 of potential, or an electric jerk, the resistance of the joint suddenly 

 diminishes, transmitting a considerable current, and signalling the 

 arrival of the electric wave which may have caused the jerk. A 

 slight shake or tap is sufficient to reduce the coherer to its former 

 high resistance. 



All metals do not behave in the same way, but the majority thus 

 show an increase of coherence under electric influence, and a sudden 

 decrease under mechanical influence. A few metals (e.g. silver) 

 appear to behave in an opposite direction. 



The earliest instances of electrical cohesion exhibited by the 

 lecturer was the small vertical fountain issuing from a smooth 

 orifice, which was found by Lord Rayleigh to scatter its drops by 

 mutual collision except when they were under the influence of an 

 electric field such as that due to a piece of sealing-wax held within a 

 yard or two of the place where the jet breaks into drops. Another 

 variety was the pair of horizontal jets, which, when they impinge, 

 unite or rebound according as there is or is not a difference of 

 potential between them of one or two volts. A pair of soap bubbles 

 in contact were also shown by Mr. Boys to cohere and become one 

 directly a stick of sealing-wax was produced in their neighbourhood. 



The two halves of a mercury globule on a flat surface, cut into 

 two with a greasy knife, and the parts connected to the poles of a 

 battery, were found by Lord Eayleigh and by Mr. Appleyard to 

 re-unite directly they were connected to the terminals of one or two 

 Grove cells ; a slight delay in the union suggesting that a film of 

 foreign matter was being squeezed out from between the globules 

 under the force of electrostatic attraction. 



