Sept. 15, 



52.] 



• KNOWLEDGE 



2G1 



Guards' Camp, and a battery of English guns is planted 

 on the mound of ^Maskhuta. 



It has generally been supposed that the name of 

 ■" Ramses " was given to this place by the French con- 

 structors of the line ; but at the time of Lepsius' visit in 

 1845 there was not yet a foot of railway laid down in 

 Egypt. The name of " Ramses," or " Ramsis," is, there- 

 fore, in all probability as old as the period of the Hebrew 

 sojourn. 



(To be continued.) 



THE AMATEUR ELECTRICIAN. 



ELECTRICAL IIEASUEEMEXT.— III. 



THE next point we have to consider is Conductivity, or 

 "that property of matter in virtue of which an electric 

 current is propagated through it." All matter is not 

 equally endowed with this property — that is to say, elec- 

 tricity does not pass with equal readiness through all the 

 various substances with which we are acquainted. For 

 instance, iron does not conduct, transmit, or propagate 

 electricity so readily as does either copper or silver ; nor 

 does mercury so readily as iron, nor water as mercury. 

 There being, then, a wide divergence in the relative con- 

 ducting properties of diflerent bodies, it may naturally be 

 asked — Is there any line of demarcation or any definite 

 law governing the facilities oflered for the passage of an 

 electric current 1 If 'v.e limit our inquiry to diflerent 

 masses of the same material, our answer will be in the 

 affirmative ; but no one has ytt explained why copper 

 should conduct better than iron, zinc, tin, <.tc. Xor will 

 we attempt to accomplish such a task. 



Ohm, who first formulated the law which bears his name, 

 •declared electricity to be propagated in a manner closely 

 akin to the way in which heat is transmitted, and that the 

 best way to study the laws influencing the more subtle 

 force is to investigate and apply the laws generally ac- 

 knowledged as governing thermal transmission. Since that 

 time experiments have demonstrated the truth of Ohm's 

 ■deductions. So closely, in fact, are the forces of heat and 

 •electricity allied, that the tables of co-efficients are almost 

 identical — that is to say, bodies which readily conduct 

 heat as readily conduct electricity, and vice verstl ; and 

 the same relation which exists between the thermal con- 

 ductivities of diflerent bodies, exists also between their 

 electrical conductivities. Conductivity is a property 

 inherent in all substances, but in different degrees — 

 that is to say, every substance conducts a greater or less 

 amount of electricity. There is, however, as is well 

 known, a vast diflerence between the best and poorest 

 conductors. Thus, pure copper conducts G,75i million 

 times as well as distilled water, and 16 million times as 

 ■well as sulphuric acid. Water, again, is an infinitely 

 better conductor than air, ebonite, sulphur, and very 

 many other substances. Practically, the conductivity of 

 such bodies is nV. The almost total absence of this 

 property in some forms of matter renders them of vital 

 importance to tlie electrician. The converse of conduc- 

 tivity is resistance, or, in other words, bodies which 

 conduct well ofler but little resistance to the passage of 

 a current of electricity, and bodies v.hich conduct but 

 poorly do so by virtue of the greater resistance which 

 they ofler to the passage of the current. Indeed, 

 as every substance offers a greater or less resistance, 

 we can with advantage start witli this property of matter, 

 and classify substances rather in the order of their resist- 

 ances than of their conductivities. So, then, we measure 

 the transmitting value of a medium, not by the amount of 



its conductivity, but of its resistance. And in measuring 

 resistance, it will be seen that we at the same time measure 

 the work which a current of electricity performs in over- 

 coming that resistance. Wc require, however, a standard 

 of measurement, and what this standard or unit shall be 

 is a question which has caused many heart-burning con- 

 tentious, and which is not yet quite settled. The unit 

 which has long been in use in England, which has, on 

 account of its eminently scientific origin, been almost uni- 

 versally accepted, and which will soon be stamped with the 

 authority of an International Congress, is known as the 

 Ohm, or B. A. unit. We will not attempt to define its 

 origin or derivation, but say simply that an Ohm is the 

 unit of resistance, and is equal to that oflered by 18i inches 

 of copper wire, 004 of an inch thick (about number 40 

 gauge). 



Resistance varies as the length of the wire, and 

 inversely as its section, that is to say, in the first place 

 that if a mile of wire oflers a resistance of 2-5 Ohms, 

 ten miles of the same wire will ofler ten times the resist- 

 ance of 250 Ohms ; and, in the second place, if we use a 

 wire so much thicker or larger as to double its sectional 

 area, we halve the resistance. For wires of uniform 

 make, it is evident that with equal lengths the sectional 

 area varies directly as the weight. The effect of tem- 

 perature upon resistance is very important. With metals, 

 the resistance increases with the temperature, so that a 

 piece of platinum through which an electric current is 

 passing increases in temperature and resistance, sympa- 

 thetically, until, if the wire is not too thick, the heat over- 

 comes the cohesion of the particles, and the wire breaks. 

 With partial conductors the reverse is the case. This is 

 especially noticeable in the carbon filaments of the various 

 incandescent lamps. The Edison filament, which, when 

 cold, oflers a resistance of about 180 Ohms, offers only 100 

 Ohms when heated to its full illuminating power. Nor is 

 this feature unimportant in its influence on practical tele- 

 graphy. The waters of the Indian Ocean are generally 

 very warm, and in consequence of this, the insulating 

 properties of the pereha in the cables is very sensibly 

 reduced, their efficiency for telegraphic purposes being 

 necessarily reduced in proportion. There are in general 

 use two methods of measuring resistance, viz., the 

 "Bridge," and the " Diflerential.' These it will be our 

 duty to consider very soon. 



THE TREE-TOAD.* 



By Mary H. Hinckley. 



A RECORD of several seasons gives the appearance of 

 l\. Ilijla versicolor in the spring, in ililton, ilassa- 

 chusetts, from about the 1st to the 10th of May. Tadpoles 

 of this species I have found most abundant in the water of 

 small, still, shadowy ponds near large trees. The eggs are 

 attached singly and in small groups for a distance of one 

 or two yards along the grasses which grow up and rest on 

 the water. Unless the grass is parted they are not 

 readily seen. The gelatinous substance surrounding the 

 eggs is exceedingly thin. When tiret laid they are of a 

 drab colour on the upper surface, which becomes lighter 

 after a few hours in the water. The under surface is 

 wliite ; the extent of this colour varies ; in some cases 

 only a spot of drab is seen on an otherwise white egg. The 

 period of egg-laying, according to my observations, extends 



• Abstract of a piipcr published in tho " Procoodings of the 

 Boston Society of Xntural Uistory," Vol. sxi , Nov. 17, ISSO, from 

 the American Xaturaliat. 



