526 



NATURE 



[Sept. 12, 1S78 



down, the loss in extracting and fusing the silver was i -38 

 per cent. 



Chloride of silver is charged at three-quarters of the 

 price of silver ; and the allowance made by Messrs. 

 Johnson and Matthey for returned silver is about 6 per 

 cent, less than the price at which they sell it. The 

 charge for fusing and casting the chloride is \d. per rod. 

 Hence it appears that though the prime outlay is con- 

 siderable, the cost of renewal is small, and a battery of 

 these elements represents a certain amount of capital 

 which might be realised at any moment. The labour 

 required for setting up this battery is comparatively very 

 little, and, this done, the element, from the insolubility 

 of one of its electrolytes, is capable of standing idle for 

 any length of time without other detriment 

 than increase of internal resistance ; this in- 

 crease, too, occurs but slowly, many months 

 elapsing before it rises from, say, 5 to 20 or 

 30 ohms per cell. Moreover, the element is 

 notably clean and compact — a case 141 cms. 

 high, 107 cms. wide, and 43 cms. deep, will 

 hold 1,200. The details carefully given by 

 Messrs. De La Rue and Miiller of their very 

 great experience of this valuable cell will 

 probably be hailed as a boon by the many 

 who have occasion to make use, of voltaic 

 batteries. 



The accompanying diagrams show some of 

 the apparatus which were specially,devised by 

 the authors to meet the insulation require- 

 ments of the high electromotive force they 

 employed. 



For instance, the ordinary form of double- 

 r-eversing-key could not be used with this 

 great battery, in consequence of the forma- 

 tion of an arc when the key is raised with 

 the object of breaking the circuit. Figs. 2 

 and 3 show the new form devised by the 

 authors. H is the handle fixed to the ebonite 

 axis, which has metallic collars A z at its 

 extremities ; these are connected to the 

 springs S S' by wires inserted in the axis. 

 The battery wires are led to A and z. The 

 standards B and B, b' and B', are respectively 

 connected by diagonal wires between the 

 ebonite plates E and e'. Fig. 3 shows the 

 open circuit position. In Fig. 2 the zinc pole 

 is connected through s to b' and the leading 

 wire N' ; the silver pole being similarly con- 

 nected to V. If now the handle H be thrown 

 over to the right-hand side of the Fig. 2, z 

 will be connected to B on the right side, 

 thence through the diagonal wire to B on the 

 left side of the figure, and to P', while Ag 

 will be similarly connected to N'. In this 

 key, on breaking contact, the arms s s' can 

 be removed from the standards B B' by a 

 distance greater than that to which the arc 

 can be drawn out. 



Fig. 4 shows the micrometer discharger, 

 by means of which the authors are able to 

 measure the length of the spark to within 

 toJou of an inch. Its construction is sufficiently obvious ; 

 it need only be remarked that the nut fixed in the cross- 

 head at the top of the frame through which the screw 

 works is in metallic communication with the clamp c, 

 and is divided horizontally into two parts, which are 

 pressed asunder by three spiral springs in order to prevent 

 shake or play of the micrometer screw. 



For special experiments it was necessary to design and 

 have constructed a commutator capable of reversing the 

 current many times in a second ; that shown in Fig. 5 

 represents the most convenient form ; it is capable of 

 reversing the current 352 times in a second when the 



handle is turned 240 times in a minute. It will be seen 

 that each revolution of A, B, c, D reverses twice. The 

 figure is so distinct as scarcely to require any description ; 

 B and D are of one piece of metal, and also A and C of 

 another, the spring conductors making contact at 90° 

 distance from each other ; each of the uprights supporting 

 the axis of the revolving disc is in metallic connection 

 with its respective insulated clamp. In the position 

 shown in the figure the positive current passes from A^to 

 the upright supporting the axis of the revolving disc, and 

 through the right hand spring to the wire plate Ag'; the 

 negative current from z to the upright on the other side 

 of the revolving disc, only partly seen, thence through the 

 upper spring to z'. 



Fig. 3. 

 FiG3. 2 and 3.— Double Reversing Key. 



Besides this, the authors had a contact breaker very 

 similar in appearance, and shown m situ in Fig. 7 on the 

 top of the dwarf cabinet of battery No. i, containing 

 1,080 powder-cells; this cabinet top is of ebonite, and 

 forms their ordinary working bench. M m' represents a 

 revolving mirror, which has a multiplying wheel, and in 

 which the reflection of the discharge in a vacuum tube 

 can be seen. In the circuit was a set of coil-resistances 

 from I to 1,000,000 ohms, specially insulated, the wires 

 running in grooves on insulating cylinders made of 

 paraffined cardboard, in order that they may be kept at a 

 distance ; besides this set of resistances there were four 



