.S7A' \V11.I.1A.M SIEMENS, F.R.S. 367 



is liiniird and discordant with the range of the spiral springs. 

 It is desirable on this account to make the diaphragm of as thin 

 and flexible metal as possible, and to make the annular indenta- 

 tions as deep as they can be made. This consideration led me to 

 try a diaphragm of silk impregnated with solution of india-rubber, 

 which diaphragm has the advantage of being more flexible than 

 one made of metal, but is liable, on the other hand, to stretching 

 under the constant pressure of the mercury. A diaphragm of thin 

 steel plate has been found to be sufficiently flexible for the purposes 

 of the instrument. 



It was desirable to avoid levers, pulleys, and other such working 

 parts in the instrument, which parts are liable to derangement 

 from stretching, bending, and abnormal expansion, which would 

 make the instrument liable to change its zero position. I have 

 therefore had recourse to a micrometer-screw with electrical 

 contact, which, with great solidity and simplicity of parts, affords 

 the advantage of a long and accurately divided scale. 



READING OF BATHOMETER. The micrometer-screw passes 

 vertically through a boss below the centre of the diaphragm, which 

 is attached to the tube by means of two insulating supports of 

 ebonite. A galvanic battery is connected through one pole to the 

 body of the tube, and by the other to the boss through which 

 passes the micrometer screw. An alarum or galvanometer is 

 comprised in the electrical circuit, which is closed whenever the 

 end of the micrometer-screw touches the extreme point of the 

 crosshead supporting the centre of the diaphragm, and therefore 

 the weight of the mercury column. The galvanometer and 

 alarum are so constructed that one element is sufficient to produce 

 the signal, as, if a number of elements were employed, discharges 

 of currents would ensue and affect the surfaces of electrical 

 contact. It is important to clean these surfaces from time to 

 time, by passing a sheet of stout paper or of fine emery-paper 

 between them. A graduated circle is provided to indicate the 

 precise angle through which the micrometer-screw is moved from 

 its zero position when its point touches the end of the crosshead, 

 an event marked by the sounding of the alarum or motion of the 

 galvanometer-needle. The points of contact on the crosshead and 

 on the micrometer-screw are made of platinum in the usual way ; 

 but the contact-piece carried by the screw is attached to the same 



