120 STRAINS IN THE HULL OF A SHIP AT SEA, 



Since the range of the instrument is quite limited it is necessary to establish 

 longer or shorter gauged lengths on the measured member according to the expected 

 magnitude of the strains in order that they shall not exceed the capacity of the in- 

 strument. On the present occasion the strains were generally small, and it was 

 expedient to use extension rods which covered a length of 6 inches on the deck 

 plates. The gauge is shown in position on the deck in Fig. 2, Plate 65, where it 

 appears spanning a lap joint, stepping down from one plate to another. Contact 

 with the deck plates is made by means of conical steel points attached to the beams 

 or to the extension rods. Sufficient roughness was found on the deck plates to 

 actuate the instrument when a weight was placed over each contact point, as illus- 

 trated in Fig. 2. The contact points would be placed in small holes made with a 

 center punch in the deck plates if needed. 



Moderate loads causing strains of a few hundred-thousandths of an inch, or 

 slowly applied loads, can be measured with comparative ease, but the effects of 

 rapidly changing loads of considerable magnitude are less easily observed. The in- 

 strument is a new one and was used for the first time on this voyage. A number of 

 modifications in its details will be made to facilitate taking readings, making it even 

 a little more compact, and providing means for attaching it to different shaped 

 members and in different positions. 



It responds promptly to changes in strains in the member under examination. 

 Stamping upon the deck at a distance of 25 feet from the gauge caused longitudinal 

 vibrations which the instrument responded to. The results here presented are re- 

 garded as close approximations to the actual strains which were developed but are 

 not held to be rigorously exact. 



A strain gauge of the type shown by Fig. 3, Plate 66, was used for measuring 

 the deck plates during the time cargo was being received. The gauge employed 

 had a working length of 20 inches. It is telescopic in design. There is an outer 

 tube which carries a conical contact point near one end of the instrument, and an 

 inner bar which carries a contact point near the opposite end. The longitudinal 

 movement of the inner bar with reference to the outer tube, and consequently the 

 movement of the contact points, is measured by a screw micrometer attached to the 

 outer member. The contact points are inserted in small drilled and reamed holes 

 made in the deck plates, and, when thus centered, readings with the micrometer are 

 taken. 



The gauge is used in connection with a steel reference bar made of the grade 

 of structural steel, and lengths on the deck are referred to this standard bar and 

 recorded in terms of it. Concerning manipulative features, check observations, 

 when the instrument is used under favorable conditions, commonly give the same 

 ten-thousandth of an inch reading of the micrometer. In engineering structures 

 in general it is not expected to define stresses much nearer than 5CXD pounds per 

 square inch, which corresponds to a strain of three and one-third ten-thousandths 

 of an inch in 20 inches length. 



The deck plates of the ship afforded a better opportunity than usually experi- 



