574 Mr. A. E. Garrett on Positive Electrification 



experiments are known to be bodies which undergo decompo- 

 sition when strongly heated, and the ionization in this case 

 may be due to chemical action. Aluminium phosphate, on 

 the other hand, is an extremely stable substance, and it is of 

 interest to investigate the source of ionization in this instance. 

 This salt was therefore chosen for detailed examination. 

 The apparatus used is shown in the following diagram. 



Fig. l. 



The glass tube consists of two parts connected by a ground- 

 glass joint. The part A is shown in vertical section, the part 

 B in horizontal section, e is the strip of thin platinum foil, 

 i sq. cm. in area, on which the salt is placed to be heated; the 

 temperature of the platinum e is raised by a current, the leads 

 for which are the thick copper wires c, d. The electrode E, 

 which is connected through a galvanometer to earth, is an 

 oblong aluminium disk about 3 sq. cm. in area; the distance 

 between the heated salt and this electrode was in most cases 

 0*5 cm. /, g are wires of the thermo-electric couple used for 

 ascertaining the temperature of the platinum foil with which 

 they are fused. To obtain the temperature from the thermo- 

 electric current, the deflexion of the galvanometer to which 

 leads from the junction pass was noted when a tiny particle 

 of K 2 S0 4 just melted on the foil. In this way the deflexion 

 for two temperatures differing by about 1000° C. was obtained, 

 and the temperature in degrees centigrade corresponding to 

 any other deflexion could be got by aid of the correction 

 curves given by Callendar *. The reliability of this method 

 was checked by observing the melting-point of Na 2 S0 4 . The 

 observations of the behaviour of aluminium phosphate were 

 taken over a range of temperature from 900° C. to 1300° C. 



Before commencing work with aluminium phosphate it was 

 ascertained that only a small current due to ions of either 

 sign could be detected when the platinum foil was used 

 alone. 



* Phil. Mag. [51 xlviii. pp. 519 et seq. 



