Feb. 3, 1876] 



NATURE 



263 



\ 



Fig. 3 shows an ingenious torsion galvanometer, de- 

 vised, we believe, by the author, and which is stated 

 to be free from many sources of error. 



An ingenious contrivance, due to the author, is shown 

 in Fig. 4. This is a so-called voltastat, an arrangement 

 whereby the current interposes " by its own greater or 

 less action a greater or less resistance in its own circuit," 

 and thus the voltastat or automatic rheostat behaves Hke 

 the governor of a steam-engine. There are of course 

 several obvious disadvantages in the use of such an 

 instrument, but we cannot recall any other continuous 

 self-acting " voltastat." Helmholtz, in order to keep the 

 disc of his siren rotating at a constant rate, employed 

 an electro-magnetic arrangement with attached governor, 

 but here the current was interrupted when it exceeded 

 a certain strength. 



In the electro-chemical portion of this treatise students 

 will find much information concerning the reactions 

 within various forms of batteries and in the electrolysis 

 of salts, and some interesting facts on electric osmose. 

 The explanation Dr. Guthrie gives of the prevention 

 of " local action " by amalgamation of the zinc is new 

 to us. The chief cause of local action — which the 

 author describes as " a coasting trade "—is attributed to 

 difference of hardness rather than metallic impurities on 

 the surface of the zinc ; mercury, it is asserted, removes 

 this inequality of hardness due to irregular cooling, " for 

 as the mercury penetrates, the mass softens and mole- 

 cular strains are relieved, and uniformity results " (p. 141). 

 And here we would note one or two minor experimental 

 statements in the book which we think it would be an 

 advantage to modify, as readers might unintentionally be 

 misled. In speaking of the electrolysis of water the 

 author states that in the first portions of the gas collected 

 the hydrogen is less than its theoretical proportion. 

 *' This is due to the ' occlusion ' of hydrogen by platinum 

 under these circumstances. The hydrogen is absorbed 

 by the platinum. Very soon, however, the metal becomes 

 saturated, and the exact combining ratio is observed'' 

 (p. 157)- This effect, we imagine, must very promptly 

 be masked by a contrary action — to which Dr. Guthrie 

 does not allude — for, except under special circumstances, 

 the amount of oxygen is perceptibly deficient in electro- 

 lysis, and, as is well known, is due to the formation of 

 ozone. And is not the following electro-chemical state- 

 ment also open to comment? — "Hydrogen, when freshly 

 liberated, has, as is well known, an exceedingly power- 

 ful reducing action. Use is made of this circum- 

 stance to protect the copper sheathing of ships, . . . . 

 the evolution of hydrogen on the copper surface de- 

 oxidises any oxidised portion" (p. 128). A student 

 might from this be led to infer that the electro-negative 

 metal in a cell would not be protected unless hydrogen 

 were evolved on its surface. Again, in describing the 

 evolution of electricity by an ordinary electric machine, 

 it is merely stated that " the -J- electricity enters the 

 prime conductor, and — leaves it ; the prime conductor 

 thus becomes -^-" (p. 52). A reference to the inductive 

 action exerted on the prime conductor, the high tension 

 at the points, and therefrom the discharge of the induced 

 opposite electricity on to the machine, seems needed 

 here. 



We must now devote a few words to one feature 



wherein the present text-book differs from most of the 

 ordinary manuals on electricity. Dr. Guthrie has sought 

 to give the reader some acquaintance with terms and 

 methods of measurement which in general are better 

 understood by the practical electrician than the science 

 teacher. Whilst ever>' such effort cannot fail to be more 

 or less useful, the present is, we regret to notice, open to 

 criticism in several directions. But as we have neither 

 the space nor the inclination to notice all the points we 

 have marked, one illustration will suffice. On pp. 225 

 and 226 we have the unit of resistance set forth as 

 follows : " Taking i second as unit of time, i metre as 

 unit of length,* and i gramme as unit of weight, an 

 * absolute ' unit of resistance is obtained by employing the 



above equations [viz., Q (or current strength) = J- — and 



I Ttl 



W (or current work) = Q^rt\ and this multiplied by 

 one hundred million is the Ohm or B.A. unit" Here, 

 irrespective of other considerations, there is the funda- 

 mental error of using the term iveight instead of mass, 

 and moreover, the student must fail to grasp the idea that 

 electric resistance can be expressed as a velocity, and has 

 nothing to do with either weight or mass. There is no hint 

 of these considerations in the manual before*us ; the elec- 

 trostatic system of units is not even referred to, nor is the 

 student made aware of the precise nature of the units 

 described. 



This text-book is open to criticism also in some other 

 portions which deal Avith more familiar questions. Notably, 

 take for example a proof connected with Ohm's law, given 

 on p. 185 ; or the paragraphs on linear resistance, § 243 — 

 247, which certainly will bewilder the reader unneces- 

 sarily, when a more general result can be deduced more 

 easily and obviously in as many lines as pages are here 

 devoted to the subject. Nor is it necessary for the par- 

 ticular proof, even if it were true, that the potential at the 

 zinc end of the battery is = o, as stated on p. 221. 



In § 257, referring to the effect of heat on the resistance 

 of hquid conductors, the fact is lost sight of that mercury 

 (quoted as militating against a theory that is given) is not 

 an electrolyte, and so has nothing in common with the 

 generality of hquid conductors. On p. 224 the author 

 shows how the diameter of fine wires may be deduced 

 from their length and weight, and then adds ; " The rela- 

 tive diameter of two wires can be deduced from their 

 weights, lengths, and resistances j " here weight, no doubt, 

 was meant to be omitted. There is also an earlier 

 paragraph needing great amendment, viz. § 214, where 

 electrical resistance is compared with the resistance 

 experienced by water in flowing through pipes ; but as 

 any analogy there might be is destroyed by the definition 

 adopted of water-resistance, it is, we think, a mistake to 

 have introduced the elaborate and withal erroneous com- 

 parison that is given. And surely two woodcuts of the same 

 tube on p. 1 79 were hardly necessar>', as if turning the 

 tube one way or the other could make any difference in 

 the reasoning. In fact, the evident care everywhere 

 taken by the author to make his meaning clear, has per- 

 haps led him occasionally to the opposite extreme of 

 unnecessarily laboured explanations, so that some really 



* After the strong reasons which exist in fav«ur of the C.G.S. system of 

 units, it is to be hoped that the centimetre will become more generally used 

 as the unit of length. 



