Science Siftings

New Zealand Tablet, 31 July 1913, Page 47

Science Siftings

By ‘ Volt.’

Ether-Telephone Problems. In attempting to estimate the probable meaning of the current epidemic of ‘ practical ’ wireless telephones, it is perhaps Well to bear in mind the fact that ‘final solutions ’ of this very attractive problem have been appearing in the news with great regularity for' the past six or seven years. As far back as 1906 a commercial system of telephoning without wires was widely heralded in the press, and since then dozens of others have enjoyed brief news honor on the front page. Not long ago a commercial wireless telephone was installed on some American naval vessels. At first sight this seemed like a good certificate of practicality. But it was not long before the announcement appeared that the apparatus had been discarded. The conclusion that it wasn’t considered worth even an effort to tinker it into form may not be justified, but is certainly suggested by the circumstances. What Cement Does. The term cement includes all those substances employed for the purpose of causing the adhesion of two or more bodies, whether originally separate or divided by an accidental fracture. As the substances that are required' to be cemented together are exceedingly various, and differ much in their properties as to texture, etc., and as the conditions under which they are placed; with regard to heat and moisture, are also exceedingly variable, a number of cements, possessed of very different properties, are required; for a cement that answers admirably under one set of circumstances may be perfectly useless in others. The different parts of a solid are held together by an attraction between their several particles, which is termed the attraction of cohesion, or cohesive attraction. The amount of this varies with the substances ; thus, the cohesion of the particles of iron to one another is enormously great, whilst that between those of chalk is small. The attraction acts only when , the particles are in the closest possible contact; even air must not be between them. If, after breaking any substance, we could -bring the particles into as close a contact as before and remove the air, they would reunite, and be as strongly connected as ever. But, in general, this is impossible; small particles of grit and dust get between them; the film of interposed air cannot be removed;* and thus, however firmly we press the edges of a broken cup together, it remains cracked china still. Perfectly flat, clean surfaces, like those of freshly-ground plate glass, may sometimes be made to cohere so that the two pieces become one, and cannot be separated without breaking. The attraction of cohesion takes place between the parts of the same substance, and must not be confounded with that of adhesion, which is the attraction of different, substances to one another; for example, the particles of a piece of wood are united by cohesive attraction, whilst the union of glue and wood to each other depends on adhesive attraction. It is important that this distinction be borne in mind, for, in almost all cases, the cohesion between the particles-of the cement is very much less than the adhesion of the cement to other bodies ; and if torn apart, the connected joint gives way, not by the loosening of the adhesive, but by the layer of cement splitting down the centre. 1 ; Hence the important rule, that the less cement in a joint the stronger it is. Domestic manipulators usually reverse this, by letting as much cement as possible remain in the joint, which is, therefore, necessarily a weak one. A thick, nearly solid cement, which cannot be pressed out .of the joint, is always inferior to a thinner one, of which merely a connecting film remains between the united surfaces.