476 REPORT — 1867. 



temperature and inconstancy when kept for any considerable time. These 

 valuable experiments have not caused anj^ expense to the Association. 



The determination of a unit of capacity has occupied Dr. Matthiessen, Mr, 

 Hockin, Mr. Foster, and Mr. Jenkin during the last two years. 



Very considerable difficulties have been encountered, and are not yet wholly 

 overcome. The methods by which both the electrostatic and electromagnetic 

 units can be determined, and midtiples or submultiples prepared, arc suffi- 

 ciently simple in theoiy, but they assume that the condensers or Leyden 

 jars compared have really a definite capacity, and that with a given electro- 

 motive force, between the induction surfaces, a definite quantity of elec- 

 tricity will be contained in the jar or condenser. This is very far from true 

 with condensers of ordinary form. "Whether the dielectric separating the 

 plates be glass, mica, gutta percha, paraffin, ebonite, or any other known 

 solid insulator, an absorption of electricity takes place ; the longer the plates 

 are charged, the more electricity the condenser will contain, and conversely, 

 it win continue to discharge itself for a very long period after the inner and 

 outer armatures have been joined. AVith some of the best insulators the 

 effect will continue for hours, if not for days. Condensers made with these 

 solid dielectrics have therefore no definite measurable capacity. This capa- 

 city will differ according to the time during which they have been charged, 

 and it may also vary with extreme variation in the electromotive forces em- 

 ployed, although this latter change has not been detected wlien the differ- 

 ences of potential are such as between one Danicll's cell and two hundred. 



Only gaseous dielectrics appear free from this embarrassing peculiarity, called 

 absorption, polarization, or residual charge. One object of the Subcommit- 

 tee has therefore been to construct condensers in which air alone separated 

 the induction-j^latcs. But new difficulties arose in carrying this idea into 

 practice. kSome support for each i)late was neccssaiy, and then leakage 

 occurred from one plate to another over the surface of any small insulating 

 supports employed, such as glass balls or vulcanite stems. It was possible, 

 by great care in drying the air, occasionally to make condensers of this type, 

 Avhich would remain insulated for a short time, or even for some months ; 

 but long experience has shown that an artificially dried atmosphere cannot 

 be conveniently maintained in any instrument which is not hermetically 

 scaled. 



Dust also accumulated between the plates of the trial-condensers ; this 

 altered their capacity and increased the leakage from plate to plate. Even 

 a single filament of dust, by springing up and down between the two elec- 

 trified surfaces, would occasionally bring them to the same potentinl with 

 great rapidity, neutralizing the charge ; moreover a condenser of this type 

 could not be taken to pieces and cleaned, for no mechanical contrivances 

 could ensure that the parts after cleaning would return to their original 

 position so exactl}^ as to constitute a condenser of the same capacitj', before 

 and after the cleaning. It is therefore clear that an air-condenser can only 

 be constructed in a hermetically sealed case, containing an artificially dried 

 atmosphere ; and even with these conditions, excluding the graduated and 

 adjustible condensers, which were fii'st tried, the air-condenser is not easily 

 constructed. For large capacities, which arc alone useful in connexion 

 with practical telegraphy, the plates require to be so iiiimerous and large 

 as to make the expense great and the b\ilk very inconvenient. 



It is hoped by the use of tin plates, soldered to metal rods, and supported 

 on insulated stems inside a soldered metal case, that these objections may be 

 partly avoided ; but meanwhile practical men have introduced condensers of 



