342 Professor Oliver Lodge [June 1, 



A handy portable exciter of electric waves is one of the ordinary 

 hand electric gas-lighters, containing a small revolving doubler, 

 i. e. an inductive or replenishing machine. A coherer can feel a 

 gas-lighter across a lecture theatre. Minchin often used them for 

 stimulating his impulsion cells. I find that when held near they act 

 a little even when no ordinary spark occurs, plainly because of the 

 little incipient sparks at the brushes or tinfoil contacts inside. A 

 Voss machine acts similarly, giving a small deflection while working 

 up before it sparks. 



And notice here that our model eye has a well-defined range of 

 vision. It cannot see waves too long for it. The powerful dis- 

 turbance caused by the violent flashes of a Wimshurst or Voss 

 machine it is blind to. If the knobs of the machine are well 

 polished it will respond to some high harmonics, due to vibrations 

 in the terminal rods; and these are the vibrations to which it 

 responds when excited simply by an induction coil. The coil 

 should have knobs instead of points. Sparks from points or dirty 

 knobs hardly excite the coherer at all. But hold a well-polished 

 sphere or third knob between even the dirty knobs of a Voss 

 machine, and the coherer responds at once to the surgings got up 

 in it. 



Feeble short sparks, again, are often more powerful exciters than 

 are strong long ones. I suppose because they are more sudden. 



This is instructively shown with an electrophorus lid. Spark it 

 to a knuckle, and it does very little. Spark it to a knob and it 

 works well. But now try this experiment : — first spark it to an 

 insulated sphere, there is some effect ; discharge the sphere, and take 

 a second spark, without recharging the lid ; do this several times ; 

 and at last, when the spark is inaudible, invisible, and otherwise 

 imperceptible, the coherer some yards away responds more violently 

 than ever, and the spot of light rushes from the scale. 



If a coherer be attached by a side wire to the gas pipes, and an 

 electrophorus spark be given to either the gas pipes or the water 

 pipes, or even to the hot -water system in any other room of the 

 building, the coherer responds. 



In fact, when thus connected to gas-pipes one day when I tried 

 it, the spot of light could hardly keep still for five seconds. Whether 

 there was a distant thunderstorm, or whether it was only picking 

 up telegraphic jerks, I do not know. The jerk of turning on or 

 off an extra Swan lamp can affect it when sensitive. I hope to try 

 for long- wave radiation from the sun, filtering out the ordinary 

 well known waves by a blackboard or other sufficiently opaque 

 substance. 



We can easily see the detector respond to a distant source of 

 radiation now, viz. to a 5 -inch sphere placed in the library between 

 secondary coil knobs; separated from the receiver, therefore, by 

 several walls and some heavily gilded paper, as well as by 20 or 30 

 yards of space (Fig. 19). 



