132 



- KNOWLEDGE ♦ 



[Aug. 31, 1883. 



is no argument against small wheels ; on the contrary, I 

 consider it is one of the strongest points in favour of them, 

 that the rider may have them geared up to any degree that 

 suits him, still keeping his machine as low in weight as 

 possible. rj.5^aj' ^ '^, 



There have been complaints, I am aware, of machines 

 with small wheels giving a painful amount of vibration ; 

 but I have in every such case traced it to another source — 

 namely, that with small wheels the makers have given 

 very small tyres, in the endeavour to iiake the machine 

 lighter with as little trouble to themselves as possible. 



Now, the smaller the wheels and the lighter the machine, 

 the larger the tyres require to be to lessen the vibration, 

 both to the machine and the rider. Small wheels must 

 have tyres at least the same size as are usually put on large 

 wheels, or they will be unfairly tested. 



Only one objection that I know of can be urged against 

 small wheels. Some persons dislike their appearance. It 

 is difficult to argue on matters of taste ; liut I think small 

 wheels look best, because with large wheels the machine 

 predominates over the rider, while with small wheels the 

 man dominates the machine. 



THE AMATEUR ELECTRICIAN. 



BATTERIES.— IV. 



IT was observed in the preceding article that the 

 Leclanche cell introduced a great peculiarity in the 

 matter of the negative element and its surroundings. The 

 utility of the Ijattery is so great that several modifications 

 in its construction have been devised. One of them is the 

 " Gravity " Leclanche. It is generally made in troughs of 

 ten cells, each cell being separated from its neighbours by 

 means of slate or other suitable material coated with 



marine glue.* Pig. 1 is an illustration of the battery. The 

 zinc plate is of a peculiar shape (as shown in Fig. 2), and is 

 supported by the strap resting on the slate partition. The 

 carbon is in the form of a rectangular rod or block, which, 

 passing through the hole in the zinc plate, dips into a 

 quantity of broken carbon and manganic dioxide. Care 



* Marine glue is a substance insoluble in water and acids, but 

 readily soluble in carbonic disulphide (CS.,). Originally it was 

 composed of caoutchouc dissolved in naphtha and shellac, but the 

 cost of the ingredients is too high for general purposes, and the 

 material now known as marine glue is made from less costly con- 

 stituents. It acquires a treacley subsistency on gently heating, and 

 cools very rapidly. For such purposes as it is usually employed 

 by amateurs it may be worked with little difBculty. It may bo 

 heated in an old iron ladle over a gentle fire, and then poured out 

 as required ; or small pieces of it may be laid on the substance to be 

 coated, and an old file or other piece of iron, heated to a dull red, 

 passed over it, but not allowed to touch it until the heat is consider- 

 ably reduced. An even layer may be very easUy made in this way. 

 Some complain that they cannot work the glue, but where this is 

 the case it can only be through downright clumsiness. 



must be taken that the carbon black does not touch the 

 zinc. A layer of sal-ammoniac is placed on the zinc plate, 

 and water being added, it is gradually dissolved. A 

 copper strap cast into the plate, and serves to connect 

 it to the carbon in the next cell. 



The action is, naturally, the same as in the ordinary 

 Leclanche, the advantage gained being a considerable re- 

 duction in the resistance consequent on the omission of the 

 porous earthenware pot. The resistance is 1 '.S Ohms, the 

 E INI F being of course the same as in the ordinary form. 

 Care should be taken not to disturb the battery, nor should 

 it be forgotten that it is adWsable, if not absolutely neces- 

 sary, to leave the lid of the trough (should it have one) 

 slightly raised, to permit any free ammonia that may be 

 given off to escape. As is the case with all batteries, 

 cleanliness is essential. The salt (sal ammoniac) has a 

 strong tendency to crystallise, but a damp cloth passed over 

 the edges of the cells occasionally will suffice to prevent 

 this. 



Another modification, having for its object the omission 

 of the porous pot, but free from the difficulties attending 

 the use of the gravity form, is that known as the 

 " Agglomerate." This, too, has several varieties. It will 

 be sufficient if we confine ourselves to two of them. In 

 the first, the negative element consists of a plate or fiat 

 block of carbon placed between two blocks of compressed 

 manganic peroxide. India-rubber bands keep the three 

 blocks in contact, the porous pot being thereby dispensed 

 with. 



^f 



Fig. 3. 



The form which has met with greater favour is, how- 

 ever, of a somewhat different shape. Fig. 3 is a horizontal 

 section of the negative element which consists of a ffuted 

 block of carbon (c), having a circular rod of compressed 

 manganic peroxide (B) in each of the six flutes. A piece 

 of coarse canvas passed round and secured by a couple of 

 elastic bands keeps the parts together. In the previously- 

 described agglomerate cell the ordinary zinc rod is gene- 

 rally used, but in this one zinc cylinders are used, the 

 result being a very considerable reduction in resistance 

 and greater constancy on circuits of high resistance. The 

 resistance is but 1 of an Ohm, while as a demonstration 

 of its greater constancy it may be mentioned that a battery 

 of forty agglomerate cells has for a long time past been 

 employed on one of the busiest London and Liverpool 

 telegraph wires. It is also worthy of notice that no less 

 than 2,000 of these cells are at present in use at the 



