The following are approximate equivalents, and may be used for rough calculations: Length I millimetre (0·001)=1; inch approx. 1 gramme 1 centimetre (0-01) = Maximum Error Tolerated in Weights (Order in Council, 1907) NOTE -Although weights and measures falling within these limits are stamped, pharmacists should see that dispensing weights are accurate. CHAPTER II WEIGHING AND MEASURING The balance. The balance is an instrument for determining the relative weight of bodies; in other words, the measure of their gravitating force. The ordinary prescription balance (fig. 1) consists of a lever of the first kind, with its fulcrum in the middle. This lever is called the beam. At the extremities of the beam are suspended two pans. FIG. 1.-DISPENSING BALANCE The fulcrum consists of a hardened steel prism, called the knife-edge, which passes through the beam at right angles, and rests with its sharp edge or axis of suspension upon two supports made of hardened steel. Since two equal forces in a lever of the first kind cannot be in equilibrium unless their leverages are equal, the length of the arms ought to remain equal during the process of weighing. To secure this the pans are suspended from hooks, each of which is dependent on a knife-edge enclosed in the box ends of the beam. By this means the pans are suspended from mere points, which are practically unmoved during the oscillations of the beam. When a good balance is placed on a level plane it ought to satisfy the following conditions: 1. The two arms of the balance ought to be equal; otherwise, unequal weights will be required to produce equilibrium. To test whether the arms of the balance are actually equal, weights are placed in both pans until the beam becomes horizontal: the contents of the pans are then interchanged, when, if the arms are equal, the beam will remain horizontal; but if not, it will descend on the side of the longer arm. 2. The balance ought to be in equilibrium when the pans are empty, for otherwise unequal weights must be placed in the pans to produce equilibrium. It must be borne in mind, however, that the arms are not necessarily equal even if the beam remains horizontal when the pans are empty, for this result might be brought about by giving the longer arm the lighter pan. 3. The beam being horizontal, its centre of gravity ought to be in the same vertical line with the edge of the fulcrum, and a little below the latter, otherwise the beam will not be in stable equilibrium. Further, the beam should be perfectly rigid, of moderate length, and as light as is consistent with strength. In many dispensing establishments the hand balance (fig. 2) is preferred for general use. Its advantage depends upon the rapidity with which weighings may be made, as well as the fact that, owing FIG. 2.-HAND BALANCE. to its portable form, such a balance may be carried from one place to another for the purpose of making a weighing, instead of bringing all substances to the balance. The hand balance should be made of the best materials, the beam about 7 inches long. The knife-edges and rings at the ends of the beam should be contained in boxes to protect them from injury. Dispensing balances of this kind are usually provided with nickel-plated pans. The greater number of medicinal substances may be weighed directly on these pans without injury, but such chemicals as iodine, phosphorus, mercuric chloride, etc., should be weighed on glass. For this purpose it is convenient to keep a pair of accurately balanced watch-glasses of such size as will lie comfortably on the metal pans. Some pharmacists prefer balances having one of the pans made of glass. The objection to the latter is its liability to chip when tapped upon the edges of mortars and glasses, necessitating frequent adjustment. If money is no great object, the pans may be made of platinum, as this metal is attacked by very few chemicals in the cold. The manner of holding the balance is shown in fig. 2, the tassel being grasped between the thumb and forefinger of the left hand, the other fingers resting against the frame to ensure steadiness. Hand balances should only be raised a short distance from the counter, and on no account should the practice of raising the balance to the level of the eye be encouraged. For weighing quantities of less than 2 grains a more sensitive balance than either of the foregoing is necessary. Fig. 3 shows a very sensitive type of balance constructed on the Dutch system. The bearings are of hardened steel working on agate planes, and the instrument is provided with a steel pointer and index. Such a balance should weigh to the tenth of a grain. For weighing large quantities of material, balances of stronger construction are required. The forms in most common use are known as 'inverted' or counter machines, and possess more than three bearings. When the load does not exceed 1 to 2 lb. the pattern shown in fig. 4 finds many applications, while fig. 5 is designed for loads up to 28 lb. Stamping of trade balances.-Every weighing instrument used in trade is required to have the manufacturer's name as well as the maximum load marked in some conspicuous place. The maximum |