1894] 



on the Work of Hertz. 



333 



existed, by a creature that developed a sensitive cavity on its skin ; 

 a creature which never so much as had a name to be remembered 

 by (though perhaps we now call it trilobite). Then, in recent 

 times we recall the photographic plate and the thermopile, with its 

 modification, the radiomicrometer ; also the so-called bolometer, or 

 otherwise-known Siemens' pyrometer, applied to astronomy by 

 Langley, and applied to the detection of electric waves in wires by 

 Kubens and Bitter and Paalzow and Arons. The thermal junction was 

 applied to the same purpose by Klemencik, D. E. Jones and others. 



Fig. 14. 

 9 



« 



Bjerknes' Apparatus, showing (1) a Hertz vibrator connected to an 

 induction coil ; (2) a nearly-closed circuit receiver properly tuned 

 with the vibrator ; and (3) a one-sided electrometer for inserting in 

 the air-gap of 2. The receiver is not provided with knobs, as shown, 

 but its open circuit is terminated by the quadrants of the electrometer, 

 which is shown on an enlarged scale alongside. The needle is at 

 zero potential and is attracted by both quadrants. By calculation 

 from the indications of this electrometer Bjerknes plotted the curves 

 1, 2 and 3 on page 329. Fig. 1 represents the oscillations of the 

 primary vibrator, rapidly damped by radiation of energy. Fig. 2 

 represents the vibrations thereby set up in the resonating circuit 

 when the two are accurately in tune ; and which persist for many 

 swings. Fig. 3 shows the vibrations excited in the same circuit 

 when slighly out of tune with the exciter. A receiver of this kind 

 makes many swings before it is seriously damped. 



And, before all these, the late Mr. Gregory, of Cooper's Hill, 

 made his singularly sensitive expansion meter, whereby waves in 

 free space could be detected by. the minute rise of temperature they 

 caused in a platinum wire, a kind of early and sensitive form of 

 Cardew voltmeter. Boys, Briscoe and Watson developed this method. 



Going back to the physiological method of detecting surgings, 

 Hertz tried the frog's-leg nerve and muscle preparation, which to the 

 steadier types of electrical stimulus is so surpassingly sensitive, and 

 to which we owe the discovery of current electricity. But he failed 

 to get any result. Bitter has succeeded ; but in my experience, 

 failure is the normal and proper result. Working with my colleague, 



