Electrostatic Field produced hy Magnetic Induction. 469 



In comparing any theoretical formula with experiment, we 

 must not forget that what we learn directly from the latter is 

 the difference of radiations at two temperatures. 



One more remark in conclusion. If the complete radiation 

 for a given temperature be represented by (5), it follows that 

 temperature may be defined by the value of a. The contrary 

 would imply that the law of distribution is the same at all 

 temperatures, and would be inconsistent with ordinary obser- 

 vation respecting "red" and "white heats." Now the 

 dimensions of a are those of a time ; so that temperature may 

 be defined by a time^ or (through the velocity of propagation) 

 by a line. Thus in Prof. Langley's curves, which represent 

 the distribution of energy in a diffraction spectrum, the wave- 

 length corresponding to the maximum ordinate may be 

 regarded as a linear specification of the temperature to which 

 the curve relates. 



Terling Place, Witliam, Essex, 

 April 24, 1889. 



LIV. On an Electrostatic Field produced by varying Magnetic 

 Induction. By Dr. Oliver Lodge*. 



IN common no doubt with many others, I have long wished 

 to find some connexion between static electricity and 

 magnetism. My early notions in this direction were such 

 ideas as the followinff : — 



(1) To spin a long bar-magnet on its axis, suspending in 

 its field a sort of quadrant-electrometer needle charged oppo- 

 sitely at either end, and to look for a deflexion. The iron 

 core of an electromagnet was supposed to serve. 



(2) To construct a cylindrical or drum-shaped horseshoe- 

 magnet like two iron wheels joined at the nave, and wound 

 with wire there ; to spin this on its axis and suspend a charged 

 pith-ball between the two rims f. 



(3) To use a varying magnet instead of a moving one 

 [quite probably this was suggested by Prof. Fitzgerald in a 

 conversation I had with him some ten years back] ; for 

 instance, to suspend a charged gold leaf between the edges 

 of the pole-pieces of an electromagnet, and watch it through a 

 microscope when the exciting current is started or stopped. 



Many other varieties of tlie experiment are in my note- 

 books, but these are enough to show the idea. 



* Communicated by tlie Physical Society : read May 11, 1889. 

 + I find, on reference to l)r. S. P. Thompson's book, that one may 

 describe this arrangement as the field-magnet of a Mordey dynamo. 



