16 Trans. Acad. Set. of St. Louis. 



negative charge. Tlie latter goes to earth through the 

 ground wire. A metal rod, one end of which is at a 

 higher temperature than the other, if surrounded b^^ 

 air would until a condition of equilibrium were reached, 

 tend to acquire a negative charge. If the two ends of 

 such a bar or wire were united by a metallic conductor of 

 different material, having the same properties but in a 

 different degree at the same temperatures, we have the 

 conditions for a tliermo couj^le. The phenomena are un- 

 der such conditions observable at much lower temperature 

 than is possible when ionization of the air or action 

 through an air space must be utilized for detection of the 

 phenomena. 



Each wire serves as a metallic conductor to connect the 

 terminals of the other. Each wire readily drains fromi 

 the other the fluid which is only imperfectly drained from 

 air. The effect is, however, in general a differential one. 

 The physical properties which determine the values v' 

 are probably involved. 



EXPLANATION OF PLATES. 



Plate I. — Fig. A. Camera photograph of brush discharge in open air. 

 Canal ray through hole in copper plate. Oscillations in the negative ap- 

 proach line. Fig. B. Same with oscillations in the positive approach 

 line. Fig. C. Dark discharge. No oscillations. Flash light after the 

 electrical exposures. 



Plate II. — Fig. A. Shorter camera exposure. Positive knob as in 

 Plate I, Fig. B. No flash light. Fig. B. Same followed by flash light. 

 Figs. C and D. End view of positive knob terminal. 



Plate III. — Fig. A. Brush discharge. Canal rays through hole in 

 copper plate. Fig. B. Hole in the copper plate is opposite the negative 

 knob. Fig. C. Hole in the copper plate is displaced. An insulated disk 

 of copper in the canal ray. 



Plate IV. — Fig A. Disruptive discharge along canal ray, through hole 

 in the copper plate. Fig. B. Same with a thin sheet of paper hung over 

 the copper plate, cutting off the canal ray. Fig. C. Hole in the copper 

 plate opposite the negative terminal, as in Fig. B, Plate III, but with 

 disruptive discharge. 



Plate V. — Fig. A. Disruptive discharge to edge of copper plate, mo- 

 mentum effects at the sharp angles in the discharge. Figs. B and C. 

 Glow discharge over photographic film between pin-heads, due to a min- 

 ute spark near the machine. Crossing of drainage lines from the ground 

 and radial discharge lines from negative pin-head. 



Plate VI. — Enlarged copy of a discharge like Fig. A of Plate V. 



Issued February 28, 1911. 



