1915] Electrons and Heat 



WEEKLY EVENING MEETING, 



Friday, May 7, 1915. 



Colonel E. H. Hills, C.M.G. R.E. D.Sc. F.R.S., Secret^ 

 Vice-President, in the Chair. 



Peofessor 0. W. PacHARDSON, M.A. D.Sc. F.R.S. 



Electrons and Heat. 



When electrified bodies are heated they are found to lose the power 

 of retaining an electric charge. The charge leaks away from their 

 surfaces. This is not a novel phenomenon. It has been known for 

 nearly two centuries that solids glowing in air are capable of dis- 

 charging an electroscope. Thus you observe that the electroscope is 

 at once discharged when I bring near it a red-hot poker withdrawn 

 from the furnace on the lecture-table. These effects are due to the 

 emission of ions by the hot solids. For example, if the electroscope 

 is negatively charged, it draws positive ions from the hot poker, and 

 so becomes discharged. 



Most bodies when heated in air at low temperatures emit only 

 positive ions. At sufficiently high temperatures ions of both signs 

 are emitted simultaneously. We can show this by a simple experi- 

 ment in w^hich the hot body consists of a loop of platinum wire and 

 acts as its own electroscope. When a charged rod is brought near 

 the loop a charge of opposite sign is induced on the latter, which is 

 thus deflected owing to the electrostatic attraction of the rod. When 

 the loop is cold this happens whatever the sign of the charge on the 

 rod. If the wire is at a dull red heat it can only be deflected by a 

 positively charged rod. When a negatively charged rod is brought 

 near it the emission of positive ions causes the induced positive 

 charge at once to stream away. Thus the wire is incapable of retain- 

 ing a positive charge, and so no deflection is produced by a negatively 

 charged rod. At very high temperatures you observe that the loop 

 is undeflected whatever the sign of the charge on the rod. The wire 

 is now liberating both positive and negative ions, and so is unable to 

 retain either a positive or a negative charge. 



If these effects are investigated in a vacuum, instead of in air at 

 atmospheric pressure, it is found that the emission of positive ions 

 gradually disappears with -continued heating, so that a wire which 

 has been well glowed out in a vacuum emits only negative ions in 

 appreciable quantity. Thus, if we repeat the last experiment with 



Vol. XXI. (No. 109) 2 G 



