TRANSACTIONS OF SECTION G. 879- 



operatioDS, instead of the faint contact which would be given, for example, by the 

 light and slowly moving hand of a metallic thermometer. 



The other trouble with previous methods was that the contact points would 

 corrode, and, in consequence of such corrosion, the instrument would fail to send 

 impulses. Corrosion of the contacts is due to breaking the circuit slowly on a 

 small surface. This is entirely remedied by breaking the circuit elsewhere than the 

 primary contact, using a quick motion, and also by giving this breaking contact 

 large surface and making it firm. 



The instrument, as applied to a thermometer, is made as follows: — From the 

 free end of the light spiral of a metallic thermometer fixed at the other end, an 

 arm is attached the end of which moves over an arc of a circle when the tempera- 

 ture varies. This end carries on either side of its extremity platinum contacts 

 which, when the thermometer is at rest, lie between two other platinum points 

 carried on radial arms. Any variation in temperature brings a point on the 

 thermometer arm in contact with one of these points and thus gives the initial 

 start to the series of operations without opposing any friction to the free motion of 

 the instrument. 



The first result is the closing of a short circuit round the initial point of 

 contact, so that no current flows through it. Then the magnets which operate one 

 set of pawls come into play. The two contact points are attached to a toothed 

 wheel in which the pawls play, and these pawls are so arranged that they drive the 

 wheel whenever moved by their magnets : thus the primary contact is broken. 



In the receiver there is a similar toothed wheel bearing the hand of the 

 indicating instrument, and actuated at the same moment as the transmitter. The 

 primary contacts are so arranged that the contact is made for each degree of 

 temperature to be indicated. This series of operations leaves the instruments 

 closed, and the pawls home in the toothed wheel. To break the circuit another 

 wire and separate set of contacts are employed. These are arranged on the arms 

 carrying the pawls, and so adjusted that no contact is made until after the toothed 

 wheel has moved a degi-ee, when a circuit is closed and a magnet attracts an 

 armature attached to a pendulum. This pendulum after starting breaks the circuit 

 of the magnets which hold the pawls down, as well as of the short-circuiting device. 

 As the pendulum takes an appreciable time to vibrate, this allows all the magnets 

 to drop back, and breaks all circuits, leaving the primary contacts in the same 

 relation as at first. 



The many details of the instruments are carefully worked out. All the contacts 

 are ruljbing contacts, thus avoiding danger from dirt, and they are made with 

 springs so as not to be affected by jar. 



The receiving instruments can be made recorders also by simple devices. Thus, 

 having only a most delicate pressure in the primary instrument, a distinct ink record 

 may be made in the receiver, even though the paper be rough and soft. 



The method is applicable to steam-gauges, water-level indicators, clocks, 

 barometers, &c., in fact to any measuring instrument where a moving hand can be 

 employed. 



TUHSDAY, SEPTEMBER (5. 



The following Report and Papers were read : — 



1. Report of the Committee on the Endurance of Metals under repeated 

 and varying Stresses. — See Reports, p. 424. 



2. On the Resistance of Stone to Crushing, as affected by the material on 

 which it is bedded. By Professor W. C. Unwin, F.R.S. 



Twenty years ago, it was common in experiments on crushing to bed the test 

 specimens of rigid materials like atone on lead plates, with an idea of securing 

 uniform distribution of pressure on the faces at which the crushing pressure is 

 applied. The author has long had the opinion that to support blocks for crushing 



