152 



Popular Science Monthly 



tive battery groups are shown in Fig. 

 3, 4 and 5. For all ordinary work, the 

 method of connections in Fig. 3 is em- 

 ployed, where the voltage of the battery 

 is four times as great as that of a single 

 cell. If the same four cells are grouped 

 as in Fig. 4, the voltage will be but twice 

 that of one cell, but the strength of the 

 current will be twice that of Fig. 4. 

 When arranged in parallel, as in Fig. 5, 

 the E. M. F. will be equal to that of 

 one cell, and the current four times that 

 obtained when the cells are connected in 

 series. The voltage of a number of cells 

 in series is equal to the voltage of one 

 cell multiplied by the number of cells. 



The voltage obtainable from any cell 

 is independent of its size or of the dis- 

 tance between the plates. For any given 

 cell, however, the current is directly 

 proportional to the size of plates, and 

 inversely to the distance between them. 

 Thus, the distance between the plates 

 affects the current only, as it increases 

 the internal resistance. 



The resistance of a number of cells 

 in series is equal to one cell's resistance 

 multiplied by the number of cells. When 

 arranged in parallel the total resistance 

 is equal to the total resistance of one of 

 the cells divided by the number of cells 

 in parallel. 



Primary cells are divided into two 

 classes. One class is suitable for con- 

 tinuous work only, and will quickly run 

 down unless connected in the circuit; this 

 is the closed-circuit type. The other will 

 rapidly deteriorate when continually used; 

 this is the open-circuit type. 



The best known of the closed-circuit 

 type is the gravity cell, shown in Fig. 2. 

 The positive pole, or cathode, consists of 

 copper located at the bottom of the jar, 

 and the negative pole, or anode, of zinc 

 crowfoot arranged at the top. Both are 

 immersed in a copper-sulphate solution. 

 This type is suitable only for such work 

 as telegraphy, or wherever small currents 

 are used, since the internal resistance of 

 the cell is great. 



Open-circuit cells are much more ex- 

 tensively used, including nearly all the 

 different makes of dry batteries. The so- 

 called dry battery consists of an outer 

 cylindrical cup forming the zinc electrode, 

 which is lined with thick absorbent paper 

 and packed with a pulverized manganese 



dioxide and carbon mixture surrounding 

 the central carbon rod. The whole is 

 saturated with ammonium chloride solu- 

 tion and sealed with pitch to keep it 

 from drying out. 



Apart from dry cells, the Leclanche cell 

 is most used. In this cell the cathode is 

 of carbon immersed in sal ammoniac 

 solution, and the anode is a bar of zinc 

 immersed in the same liquid, but insu- 

 lated from the carbon. Such cells can 

 deliver a strong current for a short time. 

 If left in circuit, however, they will run 

 down in a short time. These cells are 

 universally used for bell and telephone 

 work, and in places where intermittent 

 current is desired, as they consume no 

 energy when not in use. 



(To be continued) 



A Simple Method for Determining 

 Condenser Capacity 



A QUICK and easy method of calcu- 

 lating condenser capacity by simple 

 arithmetic, will appeal to all experi- 

 menters, and particularly to those of the 

 younger class who have not reached that 

 stage in the study of mathematics at 

 which they are able to handle formulae. 

 The curves shown here may be used. 

 As an example of their use, suppose it 



The curve shows that a condenser will need 

 almost 10 square inches of active dielectric 



is desired to build a mica stopping con- 

 denser of .002 mfd. capacity, and the 

 mica available for use as the dielectric 

 measures 8 mils in thickness. From the 



