Sept. 1, 1882. J 



* KNOWLEDGE ♦ 



229- 



Simbel, to the town which sprang up in connection with 

 the great rock-cut temples of Ra and Hatlior, this Pharaoh 

 gave the name of Pa-Rauieses. To a town — or perhaps a 

 new out-lying suburb — south of Memphis, he gave the 

 name of Pa-Rameses. To at least three towns in the 

 Delta, he gave the name of Pa-Rameses. And Brugsch- 

 Bey some years since discovered a hieroglyphic text which 

 shows that he even imposed the name of Pa-Rameses upon 

 the ancient and niagailicent city of Zoan. Basing his 

 opinion upon this text, Brugsch maintains that Zoan (in 

 Egyptain, Tan ; in Greek, Tanis ; in modem Arabic, San) 

 and the " Raamses " of the Bible are one and the same. 

 M. Chabas proposes Pelusium, a lonely mound in the midst 

 of a waste of marshes by the sea shore, about twenty miles 

 due east of Port Said. Others have suggested Heliopolis 

 (On), Babloon (old Cairo), and Toossoom, on the Suez 

 Canal. Lepsius and Ebers, both of whom have personally 

 inspected the locality, give their verdict in favour of a 

 mound locally known as Tel-el-Masroota, or Maskhuta, 

 better kno\vn to travellers in Egypt as the station called 

 " Ramsib," which is the last station but one on the line 

 between Cairo and Ismailiah. 



THE AMATEUE ELECTRICIAN. 



ELECTRICAL JIEASCEEMENT— IL 



FOLLOWING upon what was said in the preceding 

 article, electromocive force may be defined as " a 

 difference of electrical potential, Ln virtue of -which elec- 

 tricity is transferred from one place to another." If, then, 

 ■we connect two bodies of equal potential, there being no 

 difference between their potentials, there will be no electro- 

 motive force, and consequently no current. If we were to 

 connect two boilers with steam at a high pressure, by means 

 of a pipe, no movement or reduction of pressure could take 

 place. The pressure is there, nevertheless, and were either 

 of the boilers but partially exhausted, the energy laid up 

 in the other would dcvelope an equilibriating flow which 

 would continue until the unequal pressure no longer existed. 

 In the^battery cell a difference of potential is maintained by 

 the series of chemical changes which help to upset the equi- 

 librium as quickly as it is established. This action is well 

 described by Tyndall, who says, "An incessant effort, 

 never fully satisfied, is made to establish electric equili- 

 brium ; the incessant renewal of the effort maintains the 

 electric current." In dynamo-electric machines a difference 

 of potential is maintained by the rotation of the armature. 

 Returning to our connected boilers, it should be apparent 

 to all that the pressure in both will be the same, whatever 

 may be their relative capacities ; that is to say, if one is 

 twice the size of the other, the pressure in the smaller one 

 will be exactly the same as in the large one, but the 

 amount or weight of steam varies in proportion to the 

 capacities. 



The same principle is seen in electricity. The intensity, 

 or, more correctly speaking, the electro-motive force, of a 

 battery is solely dependent upon the sum of the differences 

 o£ potential, and is quite independent of the size, weight, 

 or shape of the plates. A little reflection will make clear 

 the fact that the force or intensity of the action, say, of 

 sulphuric acid on a single atom of zinc, is identical with 

 that exerted upon a million atoms. Discard for a moment, 

 if you like, the electrical side of the question, and regard 

 only the aftinity existing between the metal zinc and the 

 acidulated water, and the same conclusion will enforce 

 itself. The easiest way, perhaps, of making this point 

 clear is by analog}'. Let us liken the action to a battle 



between equally numerous armies, in which every com- 

 batant fights his hardest, and supposing the men on 

 each side to be all of the same strength, and tilled 

 with exactly the same desires towards their opponents, 

 it is clear that the struggle will be as strong between 

 any two opposing men as it will be between the whole 

 of the two contending armies. The numerical equality 

 assuined in this simile is warrantable, because however 

 much water we may use, a single atom of zinc can enter 

 into union with but one atom of oxygen (one of the t^vf) 

 constituents of water). What, then, is the advantage oi 

 increasing the size of our plate • If there is nothing 

 gained by so doing, a pin's point of zinc opposed to a pin's 

 point of copper should give us the highest attainable 

 intensity. The advantage gained is somewhat similar to 

 that which would result from increasing the size or capacity 

 of our boiler. We generate more electricity, a great€r 

 amount or quantitj' of it, a current capable of doing more 

 work of certain kinds. We shall see more clearly what 

 this means when we take into consideration the applica- 

 tions of Ohm's law. 



The electro-motive force imparted to a current by any 

 particular source of electricity is a matter of great impor- 

 tance, and becomes a factor requiring careful and accurate 

 measurement. The unit of E.M.F. (elejtrc-motive force) 

 is called a volt, in honour of Volta, one of the pioneers of 

 electric Science. 



A committee, appointed in Paris last year, is now occu- 

 pied in settling the method of determining the value of a. 

 volt ; but even were this task accomplished, the term in- 

 volves so many complex calculations, that before we c-ouLl 

 give a satisfactory definition, we should have to monopolise 

 two or three whole numbers of Knowledge. Approxi- 

 mately, the volt is equal to a Daniell cell in good condition. 

 Consequently, this battery is generally used as a standard 

 of E.M.F. 



Fig. 1 represents the form of standard cell used in the 

 Postal Telegraph Service. It is easily constructed, very 

 reliable, constant, and altogether may be fairly regarded 

 as the best form. In a wooden trough or case are three 

 chambers. Tlie left hand one contains water, in which a 

 zinc plate (Z) i<; suspended. The right-hand chamber 

 contains the flat, porous pot, immersed in water. A copper 

 plate (C) and sulphate of copper crystals are placed in the 

 porous pot The centre chamber (.\) is about half-filled 

 with a semi-saturated solution of sulphate of zinc, in which 

 the zinc plate and porous pot are iiun\ersed when the 

 battery is m use. A small rod of zinc (z) is placed in a 

 small compartment at the bottom of the cliamber, its 

 function being to displace any copper which may get into 

 the zinc solution, thereby keeping that solution clear, and 



