Further, the work of tuning is rendered more difficult by reason of the fact that the strings cannot so readily and correctly be pulled through the space intervening between the bar and the tuning-pin. It is debatable whether the greater rigidity and resonance offered by the “capo d’astro” method are not too dearly bought at the cost of inconvenience in tuning and deterioration of tone-quality. It is noteworthy that most manufacturers confine the “capo d’astro” bar to the high treble register of their instruments, where brilliancy and a somewhat metallic quality of tone are a desideratum.
The iron framing of an upright piano follows the same general lines of construction as have already been noted in the previous discussion of the grand. The functions of the frame are precisely the same and its construction differs outwardly but little. There are, however, certain divergencies from the grand forms that must be noted carefully. For example, it will be remembered that the form of the upright pianoforte is such that the framing has no direct and positive connection with the outside case. It is thus impracticable to consider the sides of the iron plate as homogeneous with the sides of the case, nor is it possible to give to it that consistency of form that, in the grand, results from the shaping of the plate to correspond with the curved outline of the case. This, however, is no reason why the upright iron plate should not be as strong and secure as that of the grand.
IRON PLATE FOR UPRIGHT PIANOFORTE FITTED WITH CAPO D’ASTRO BAR.
Among the various differences of detail that we observe to exist between these two forms of plate is the device adopted to secure the bearing and rigidity of the strings at their upper end deserves notice. The practice is entirely different in this respect as regards the upright, and we are therefore introduced to a new feature: the “pressure bar.” This device has superseded both the old-fashioned pinned bearing bridge and the later agraffe forms. It consists, essentially, of a bar of heavy metal that is screwed into the wrest-plank through the metal plate and is placed so that the strings pass under it on the way to the tuning pins. In fact it occupies the same position as do the agraffes or capo d’astro bar on the grand. Its function is also similar; namely, accurately to delimit the speaking lengths of the strings at the upper end and to assist in the formation of a thrust or bearing whereby the latter may more advantageously be secured.
IRON PLATE FOR UPRIGHT PIANOFORTE WITH AGRAFFES (MEHLIN PATENTS).
This form of building the bearing bridge possesses important advantages. It does not require to be cast into the plate, nor is it weakened, as in the agraffe system, by being broken up into a large number of units. Moreover, the bearing for the strings is formed much more smoothly and easily between the pressure bar and its attendant ribs on the plate than is possible when either the agraffes or the capo d’astro bar are used. This is an important point, for such construction tends to make the task of tuning much more rapid and correct.
There are, however, several points to be considered in the designing of pressure bars. As a general thing, it is impracticable to apply this form to the bass strings on account of the fact that the overstringing requires of them a slanting direction. On this account the pressure bar cannot be made properly to maintain the obliquely-running strings in their respective places. It is therefore usual to build the bass bridge after the old pinned type and thus to secure the bass strings by much the same device as is employed in the belly bridges.
As far as the designing of pressure bars is concerned, it will be found that care should be exercised in arranging the space that the bar must occupy and its position with regard to the scale ribs that are placed on each side of it and that support the strings before and after their passage under it. In general it may be said that the pressure bar must be screwed in such a position that its lower curved surface is lower than the upper surfaces of the scale ribs by about one-half of the height of the latter. It must also describe a curve that corresponds to the dip or angle of the scale and must be secured by enough screws (one between each three string-groups is usual), to ensure that the pull of the strings will have no injurious effect upon its rigidity. It is also essential that the curve into which the string is bent during its passage under the pressure-bar and over the scale-ribs be not too deep or sudden. The bearing surface of the bar should be gently rounded until very near its middle point where the string exercises its greatest pulling strain. Here the surface must be curved a little more sharply.