McClelland and M'Henry — Uncharged Nuclei. 299 



III.— Nuclei driven off from Metals by Heat. 



A series of experiments were conducted on tlie nuclei driven off by heat 

 from a platinum wire electrically heated. The platinum wire used was thin, 

 having a resistance per cm. of about 'IS ohms. The air after being filtered 

 and, if necessary, dried, passed through a wide glass tube. Through corks in 

 the ends of the tube thick copper wires were introduced supporting about 

 5 cms. of the platinum wire. A variable resistance and a sensitive ammeter 

 completed the heating circuit. The wide glass tube containing the platinum 

 wire was heated strongly in a Bunsen flame before being used, to expel all the 

 nuclei from the glass. This precaution was, perhaps, unnecessary owing to 

 the width of the glass tubes, whose diameter was about 6 cms., and which did 

 not become very hot in the experiments. 



The remainder of the apparatus was similar to that used in the experi- 

 ments on nuclei produced by ultra-violet light. The nuclei emitted by the 

 platinum wire were passed over a long terminal connected to earth, then over 

 uranium, and were detected at the short terminal connected to the electro- 

 meter. The mobility of the nuclei when charged was deduced from a 

 current-voltage curve as before, and gave an idea of their size. 



For simplicity, we shall first deal with the nuclei emitted when the wire 

 has been kept for some time at a dull red heat. The emission then becomes 

 very steady, and seems to be a function of the temperature alone, so that 

 when the velocity of the air through the apparatus remains constant tlie 

 number and size of the nuclei depend solely upon the current through the 

 wire. For example, if the emission is measured at a given temperature, and 

 the wire be kept then at a much higher temperature even for hours, on 

 decreasing the temperature of the wire to its former value the emission of 



Co 

 the nuclei as deducted from the ratio ^r is found to be unaltered. 



It was thought that the nuclei might have been formed by some catalytic 

 action of the platinum upon the moist air, in which the platinum itself was 

 unchanged. Whether the air was moist, however, or carefully dried over 

 calcium chloride and phosphorus pentoxide made not the slightest difference 

 in the numbers. The nuclei were, therefore, due in all probability to the 

 disintegration of the platinum itself. 



Q 



The size of the nuclei increased with the temperature of the wire. ~ was 



measured for currents of 1*2, 1'3, and 1-5 amperes through the wire, 

 the air velocity through the apparatus remaining constant. A current- 



