280 THE ELECTRIC TELEGRAPH. 



The following three determinations in the same unit will 

 illustrate this : 



1 . Copper plates in sulphate copper solution 15 



2. Amalgamated zinc plates in nitric acid 6 



3. Copper plates in nitric acid 2 



The electro-motive forces observed in these instances were 

 probably due to differences between the metal plates them- 

 selves, and not to polarisation by gas. 



In the same unit in which the above values are expressed, 

 the electro-motive force of an ordinary DanielFs element is 

 only 470, or less than that due to "polarisation by gas of the 

 platinum plates of an ordinary voltameter. It can be no 

 matter of wonder, therefore, that an element with an electro- 

 motive force so small as that of Daniell's should be found 

 incompetent to effect the decomposition of large volumes of 

 water, and that for this purpose we are obliged to employ 

 elements of greater force, such as Grove's or Bunsen's. 



"With the continuation of the current the polarisation in- 

 creases, and its amount depends, within a certain limit, upon 

 the strength of the decomposing battery ; but it attains a 

 maximum. Increase of temperature of the fluid which is 

 decomposed is followed by a decrease of the polarisation. 



38. Insulating Substances. The conducting powers of in- 

 sulating materials have been determined by various observers 

 qualitatively. The only quantitative measurements to be 

 relied upon are those made with the telegraph cables where a 

 great and uniform surface is at the command of the experi- 

 menter. Necessarily, therefore, the information which these 

 determinations afford us comprehends only a limited number 

 of materials. At a temperature of 72 F. the conducting 

 power of gutta-percha has been found to be about forty-five 

 times that of india-rubber, while at 92 F. the relation 

 between their conducting powers is almost double this. 



To calculate the conducting powers of insulating materials 

 from their resistances as dielectrics of cables, we must sup- 

 pose the propagation of the electric current from the central 

 conductor through the insulating covering to take place in 



