412 



SCIENCE 



[N. S. Vol. XLIII. No. 1108 



A round metal rod or tube, B, in the dia- 

 gram, about one centimeter in diameter, is 

 mounted in earthed metal guides which are 

 concentric with the hollow cylinder, 0. One 

 of these guides passes through the wall of the 

 wire cage, and is in metallic contact with it. 

 A hole is cut in the cage opposite the other 

 end of the hollow cylinder, so that the rod can 

 be pushed concentrically through the hollow 

 cylinder without touching its walls. The rod 

 is thus always in contact with the cage which 

 forms the earthed hollow conductor, and the 

 part of it within the hollow cylinder is also 

 within this earthed hollow conductor and in 

 metallic contact with its walls. 



Before beginning an experiment, the needle 

 of the electrometer, which was suspended by 

 a quartz fiber, was charged from 200 dry cells 

 and then insulated. The hollow cylinder, C, 

 was then put in contact with the outer cage 

 so that the free charge induced upon it by 

 the electrometer needle might be taken off. 

 "When, now, the earthed rod was pushed 

 through the cylinder, a charge was induced 

 upon the cylinder which varied with the metal 

 of the rod. Thus when a compound rod con- 

 sisting of rods of the same diameter of zinc 

 and copper put together, end to end, was 

 pushed through the cylinder, the electrometer 

 needle was differently deflected according as 

 the zinc or copper part of the rod was in the 

 cylinder. Thus in one experiment the copper 

 part of the rod was pushed through the 

 cylinder C, which was then discharged to the 

 cage and again insulated. The zinc part of 

 the rod was then pushed into C, and the 

 electrometer showed a deflection of 12.5 scale 

 divisions. This was repeated regularly many 



times. When the whole rod was withdrawn 

 from the cylinder and an insulated copper rod 

 of the same diameter was substituted for it 

 and was alternately connected to the zinc and 

 the carbon of a single dry cell, the electrom- 

 eter gave a difference of scale reading of 35 

 scale divisions. Since the electromotive force 

 of the dry cell used was about 1.25 volt, the 

 difference of deflection for the zinc and copper 

 ends of the rod indicated a difference of elec- 

 tric state of about .4 volt, the zinc being 

 electropositive to the copper. This difference 

 remained unchanged when the rod was in con- 

 tact with the outer cage on both sides of the 

 cylinder 0. 



By substituting an induction cylinder only 

 2.5 cm. long for C, it was found that the zinc 

 was most electropositive next to the copper, 

 and that its electropositive charge decreased 

 gradually with the distance from this junction. 



It has been known since the experiments of 

 Cavallo that the contact charges of two metals 

 depend upon their temperature. Since the 

 contact charges which have been observed 

 from Bennett's time on are apparently the 

 bound charges induced by the two metals upon 

 each other when close together, it was to be 

 expected that the charges which metals hold 

 while in contact with the earth or with the 

 inside of a hollow conductor should vary with 

 the temperature of the metal. By heating one 

 section of a rod of a single metal and cooling 

 another section, this expectation was verified. 

 Thus, in the case of iron, steel, copper, brass 

 and tin, the warmer part of the rod was 

 electronegative to the colder part; in aliunin- 

 ium the warmer part became markedly electro- 

 positive, while in zinc the change was very 

 slight. 



Since the Thomson effect in iron indicates 

 a change in the direction of the electromotive 

 force at the junction of a hot and a cold part 

 at about 150 degrees, an attempt was made to 

 heat one end of a steel tube and keep the other 

 end cold and measure the change of induced 

 charge with a change in temperature. It was 

 found that the tube used became more electro- 

 negative as its temperatm-e increased up to 

 150 degrees, or more. From 150 to 200 de- 



