254 Sig. Quirino Majorana on the Contact Jlieory. 



and if the electromotive force of contact between zinc and 

 copper be 0'8 volt, we have 



T _ 1327x20x0-8 _ . 8 



27rx0-25x9xl0 11 ~ i ' 5XiU * 



The deflexion of the galvanometer was 6 or 7 times greater 

 than in the former apparatus. 



Some observations must be made concerning the conditions 

 for success in the above experiment. It is very important 

 to have the surfaces of the different metallic plates in good 

 condition ; that is to say, the zinc must be freshly polished, 

 and if possible a bright mirror. If the copper is not in like 

 condition the experiment does not thereby suffer, in fact, the 

 deflexions obtained are greater with pieces of copper oxidized 

 hj heating in a gas-flame (Kelvin). 



The insulation, wherever it occurs, should be made with 

 ebonite. Pieces of wood, although dried in an oven, used in 

 the construction of similar apparatus, gave bad results ; the 

 phenomena could, however, be observed. The pulley which 

 communicates its motion to the drum must be of wood or 

 preferably of metal ; pulleys of ebonite become electrified by 

 the friction of the driving cord and completely mask the 

 phenomena. If these precautions be attended to, no other 

 disturbing causes need be feared ; thermoelectric and electro- 

 magnetic actions cannot give any trouble because the gal- 

 vanometer circuit is always open. 



The pieces of apparatus described, beside permitting the 

 measurement of the contact electromotive force between two 

 different metals, are well adapted to exhibit in a simple 

 manner the existence of this force, or at any rate the existence 

 of a difference of potential between the two metals. It is 

 only necessary to use a galvanometer of great sensitiveness. 

 Consider the first of the two forms of apparatus described. 

 The expression for the current produced in it contains in the 

 denominator the distance between the fixed and moving 

 armatures. In order to increase the current it is therefore 

 necessary to diminish this distance. Unfortunately, this can 

 only be done to a certain limit, for mechanical reasons. If 

 the distance were reduced to 001 cm., the current would 

 be 100 times greater. 



In each case the current is obtained as a transformation of 

 the work done in overcoming the mutual attractions of the 

 electric charges on the various parts of the instrument. 



