172 SECONDARY ELECTROMOTIVE PHENOMENA IN 



battery-circuit from the galvanometer-circuit, so as to ensure that 

 even when very large electromotive forces are used, such as that of 

 a Grove's battery of fifty elements, the galvanometer remains un- 

 affected when both circuits are open. I have already given shortly 

 the means by which this can be accomplished in the description of 

 my experiments on external polarisation in moist conductors l . It 

 consists in breaking both circuits in two places. The contrivance 

 by which this is effected fulfils at the same time another purpose. 

 It enables the observer to determine the time ('closing time') 

 during which the polarising current is sent through the object of 

 polarisation. I possess mechanical means by which I can alter this 

 by continuous gradation from' T ^Vo" to rV f a second, and then go 

 on at intervals of 0*3 second up to 20 seconds. Beyond this the 

 variations may be made by the watch. With the same apparatus 

 I am able to determine the interval between the opening of the 

 battery-current and the closing of the galvanometer-current. This 

 I call the ' transmission time ' (Ubertragungszeit). 



The galvanic apparatus used for polarisation was either a DanielPs 

 battery, of which the elements were of the usual drinking-glass 

 size, and the specific gravity of the dilute acid was 1*030 at 19-4 C., 

 or a Grove's battery of the small elements I use, of which I possess 

 fifty 2 . .In order to be independent of the variable resistance of the 

 pads, muscles, &c., the good condition of the battery was controlled 

 by means of a metallic closure. When other electromotors were 

 used, it is specially stated. 



The secondary electromotive actions were observed with the aid 

 of the aperiodic galvanometer. As these actions are often in two 

 opposite directions one after the other, the aperiodicity of the instru- 

 ment has special value. They render it unnecessary to use con- 

 trivances such as I formerly employed, by which the galvanometer- 

 circuit was closed after a longer or shorter transmission time, instead 

 of, as now, immediately after the opening of the primary circuit. 

 Finally, it can be readily understood that in experiments on polari- 

 sation, as in those relating to electrotonus, it is necessary constantly 

 to watch the condition of the primary current, by means of a special 

 galvanometer. This would only be possible with the Wiedemann 



1 Gesammelte Abhandlungen, vol. i. p. 2. 



3 In the year 1849, the Academy, to which I had not then the honour to belong, 

 was good enough to place at my disposal a Daniell's battery made by Siemens and 

 Halske, of 100 elements, for this research. I soon, however, recurred to the small 

 Groves, which are alike more effectual and more convenient. 



