49Q 



NA TURE 



{Sept. 14, 1882 



Contact Makers of Delicate Action, by Prof. H. S. Hele Shaw. 

 — The author his been engaged in designing a speed indicator 

 in which it is essential to have the uniform motion of a revolving 

 disk. This di~k is subject to varying resistance, so that for 

 driving it, clockwork, even though powerful and expensive, 

 could scarcely be depended upon. It therefore seemed best to 

 employ an electro-magnet acting on a ratchet wheel and con- 

 trolled by a clock. This clock for the purpose might then be a 

 common one, with lever escapement. Upon enquiry there ap- 

 peared to be no contact maker at once, absolutely reliable, 

 suitable for continuous use, and at the same time sufficiently 

 delicate in its action. 



In an instrument of this kind for completing circuit it is 

 necessary to ensure such a close approach of the surfaces forming 

 the opposite poles as practically amounts to absolute contact. 

 With small differences of potential and without previous contact, 

 the exact distances across which the current will flow appears to 

 be at present unknown. According to Prof. Guthrie the terminals 

 of 50 or 60 grove cells maybe brought to within i-toooth of an 

 inch without any indication of the passage of a current. Prof. 

 Tyndall says that a battery of more than 1000 cells is required 

 to cause a spark at i-ioooth of an inch, and Prof. Sylvanus 



Thompson mentions i-io,oooth of an inch as the distance. To 

 ascertain the quantity exactly, appears impossible without the use 

 of an instrument of such refinement as Whitworth's millionth 

 measuring machine, a modification of which might be very 

 appropriately employed for the purpose. The distance no doubt 

 is extremely small, and the consequent difficulty in the way of 

 ensuring electrical contact when the opposite poles are the sur- 

 faces of two solids, seems to merit a brief consideration. 



The ways in which two such surfaces are brought together may 

 for convenience be divided into (1) a relative motion of the two 

 terminal surfaces, normal to both ; (2) a motion tangential to 

 both ; (3) a motion compounded of these two. The first mode 

 is very common with rapid contact makers, such for instance of 

 those with electric bells and telegraphic transmitters, but these 

 require an appreciable amount of force to work them. That 

 contact makers of this kind are not suitable with delicate clocks 

 is the testimony of every clockmaker to whom the author has 

 spoken One of these instruments of very delicate construction 

 required the weight of nearly one gram to ensure electrical 

 contact, and then could not be entirely relied upon. There is 

 1 no doubt that when two surfaces of solids touch only lightly the 

 I area of contact is exceedingly small. This area may be made 



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to increase by increasing the pre- sure with which they are brought 

 into contact. The conditions of the problem under consideration 

 do not admit of this being done to any appreciable extent. 

 Thus, even when chemical compounds are prevented from 

 forming on the surfaces, dust and panicles floating in the air 

 are liable to become deposited between them, and render contact 

 uncertain. 



For these reasons contact of the second kind which is known 

 as rubbing or sliding contact has been largely adopted. This 

 action may be made perfectly reliable, and is suitable where the 

 motion is not rapid as for instance with switches and com- 

 mutators. But considerable energy may be absorbed in over- 

 coming friction. Those clockniakers who employ it, appear to 

 do so only for large clocks. The third method has, as far as the 

 author is aware, been adopted in only one way, though in that 

 way with eminent success. A contact maker of this kind is used 

 by Mr. Hargreaves of Leed-, who has had it at work for fifteen 

 years. A metal roller with rounded edge runs upon and between 

 two metal rails of circular section, thus making contact between 

 them. Contact is broken when the roller passes over a gap or 

 joint to another pair of rails. When the roller is moving along 

 the rails, there is a slight rubbing action, by reason of its resting 

 between them. This is almost certain to produce electrical 



contact, which is even fjund to occur without failure when the 

 whole is covered with dust. The metal employed is gold, which 

 with a weak current, is found to last much better than even 

 platinum. With a strong current the metal oxidises, and more- 

 over, though working admirably with a heavy pendulum, the 

 resistance is to 1 great for a delicate escapement to overcome. 



The use of a liquid terminal with which the the other terminal 

 (being solid) is brought into contact obviates most of the fore- 

 going difficulties. Of all liquids mercury is the only one which 

 can be practically employed. There are two objections to its 

 u-e which have hitherto prevented its being used for more than 

 temporary and experimental purposes: — 



1. The fact that it readily combines with oxygen on the 

 passage of an electric spark. 



2. The difficulty of rendering a contact maker of this kind 

 portable. 



The author has endeavoured to overcome the first of these by 

 causing contact to be made in a gas which has no action on the 

 mercury. The way in which this is done is as follows : A short 

 glass tube clo-ed at its upper end (such as an inverted test tube) 

 is filled with mercury, and inverted in a ves-el of mercury. The 

 mercury is then partially displaced by hydrogen gas. A very 

 light bent lever working on a pivot is connected with a platinum 



