of 50 volts causes the needle to deflect through 100 degrees of the scale, then if the instrument remains in the same condition, an electrical pressure of 50 volts will always produce that deflection. When, by a series of such comparisons, the various equivalent currents for certain deflections have been determined, the galvanometer scale may be so graduated that the pressure of the current in volts may be read directly from the instrument. Such a galvanometer might be called a voltmeter. There are, however, the same objections to its practical use as there are to the galvanometer which is calibrated to amperes; and commercial voltmeters, while depending upon the same or similar principles, are differently constructed.

Commercial Voltmeters.-A voltmeter of the same type as the ammeter in Fig. 4 is shown in Fig. 5. The principal difference between the two instruments, and between all ammeters and voltmeters in general, lies in the fact that ammeters are designed to have as little electrical resistance as possible, while voltmeters are equipped with extra resistance-coils, so as to greatly increase the resistance of the instrument. The reason for this will be explained shortly. The extra resistance-coils are contained in the back of the instrument behind the board R, Fig. 5, and consist of several sheets of cardboard (or similar material) upon which are closely wound many hundred feet of insulated thin German silver wire. This wire is all connected in series with the solenoid A by means of connections not shown, and the two free ends are brought to the binding posts or terminals, a and b. At F is a fuse which is also connected with the instrument in series and which, by melting when the current accidentally becomes too great, opens the circuit and thus saves the instrument from destruction. The course of the current in the instrument is as follows: Entering the binding post a, it traverses all the German silver resistance at the back of the instrument; then enters the fuse F, and from there flows into the solenoid. Leaving the solenoid 4, the current is carried out of the instrument by the binding post b, thus completing the circuit. It will be noticed that the solenoid A of the voltmeter is different to the ammeter solenoid shown in Fig. 4. In the voltmeter, the solenoid consists of a hollow coil of thin copper wire, having a great many turns. The action of the instrument is the same, however, as that of the ammeter: Current in the solenoid "sucks" the core in and thus causes

known that the resistance of the entire circuit (that is, of the lamps and line-conductors +C and C) is equal to 30 ohms. Let the current in the circuit be equal to 4 amperes, and the pressure of the entire circuit be 120 volts. Then, in order that these values may be indicated correctly, the measuring instruments above described must be used as follows:

I. Current Measurement. - The current supplying the lamps L must, after leaving the dynamo, pass to the lamps by way of

the conductor C, and it must return to the dynamo by the conductor C. Furthermore, to measure this current with the ammeter of Fig. 4, we know that the entire current must be made to pass through the solenoid A, which will then exert a certain pull on the core C, thus causing the needle to indicate on the dial the value 120 amperes. To allow all this current to pass through the ammeter, the instrument must be connected in series with the circuit. This is done by cutting open the circuit at a point usually near the dynamo, and inserting the ammeter in the gap thus formed. The connections having been made, every

variation of current strength in the circuit is indicated upon the dial of the instrument by the varying pull of the solenoid on the core. In Fig. 6, the ammeter, correctly connected, is shown, marked A. M. The rule for this correct connection is, therefore; as follows:

Connect the ammeter in such a manner that the entire current of the circuit is made to pass through the instrument.

II. Pressure Measurements.-The maximum pressure in any electrical circuit is equal to the sum of all the pressures which are in series with each other. Thus, in Fig. 6 the maximum pressure is equal to the pressure from the positive dynamo-terminal to the lamps, plus the pressure from the positive side of the lamps to the negative side, plus the pressure from the negative side of the lamps to the negative dynamo-terminal; or, in other words, it is equal to the pressure measured across the dynamo-terminals from +to-.

Now, in every electrical circuit, and in every conductor, the pressure is given by formula (1) namely, ECX R.

If, therefore, we have an instrument, in which R, the resistance, is absolutely constant (that is, never changes in value), then, if this instrument is connected to an electrical circuit, the current C in it will always be the same for a given pressure E; and if this pressure should rise around the instrument, the current in the instrument would rise in exactly the same proportion.

Therefore, if we know just what current in the instrument is produced by a certain pressure, we have a measure of this pressure; in other words, a voltmeter. This being the case, the pressure is measured with a voltmeter by connecting the terminals of the

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instrument around that portion of the circuit for which the pressure is to be determined. Thus, in Fig. 6, where we are desirous of measuring the total pressure, the instruments must be connected across the circuit, as shown by V. M. We therefore have the following rule for the correct connection of the voltmeter in any electrical circuit :

Connect the voltmeter in such a manner that its terminals are around that portion of the circuit in which the pressure is to be determined.

Never connect the voltmeter in series, because in that case it acts like an ammeter, and, not having the current-carrying capacity of an ammeter, it will be burned out by the excessive current passed through it.

Conclusions. From the above we see that since a voltmeter may be required to measure high pressures, as, for instance, in Fig. 6, the amount of current which could thereby be forced through the instrument would be enormous. To prevent this great quantity of current from traversing the voltmeter, a very large resistance is placed in series with it, as explained in connection with Fig. 5. The ammeter, on the other hand, which depends upon the entire current for its action, and which is subjected to a comparatively low pressure only, is designed with its resistance as low as possible, so that no pressure may be wasted in sending the current through it. Thus it is now evident that the less the resistance of the ammeter and the greater the resistance of the voltmeter, the better, as a rule, are the instruments.

PERHAPS

DERHAPS there is no subject which has called forth so much useless discussion or so many worthless nostrums as that of boiler incrustation. The great mistake made by vendors of patent nostrums for preventing or curing scale in steam-boilers is that, having found a certain compound suitable for a certain boiler, they are apt to conclude that the compound is of general utility, and they advocate it in season and out, utterly disregarding the fact that the conditions vary so much that it is difficult to find even two boilers under exactly the same conditions as to feed-water and its necessary treatment; for though the feed may be identical, it is possible that one boiler may have so much more work to do than the other that the compound which suits the lightly-worked boiler will cause foaming in the other, and, consequently, have to be abandoned.

It is therefore desirable that water be analyzed and its constituent impurities ascertained, so that a proper treatment may be accorded it. The means adopted for preventing bad effects from incrustation are either chemical or mechanical, or both. Chemical means consist of the use of substances which precipitate the solid impurities of the water and allow them to settle in some quiet part of the boiler, whence they can conveniently be blown out.

Mechanical means often depend upon the introduction in the boilers of greasy, starchy, or gelatinous substances, with the intention that as each particle of solid matter is freed from the water it shall become coated with some of the composition and thus prevented from adhering to other particles or to the boiler-plates. Large quantities of mud are formed in this way, which require, for removal, the regular use of the blow-out tap. This is a direct loss in proportion to the amount of hot water wasted by the blow-out, and is opposed to the attainment of economy in fuel combustion. It is not out of place here to inquire into the chemistry of boiler-scale. The guide taken in

this article is the treatise on the question by the late Dr. Angus Smith, who made a very careful examination of the whole subject.

He divided waters that are injurious to boilers into three main classes:

1. Alkaline, or chalk, water.
2. Neutral, or gypsum, water.
3. Acid water.

Numbers 1 and 2 are very frequently found together.

Numbers 2 and 3, also, are often found together.

Numbers 1 and 3 cannot be found together, as they would tend to neutralize each other until only one was left.

Now, all such waters are injurious if put into a boiler, the first two because they form scale, and the third because it dissolves the plates.

Chemists tell us that carbonate of lime, which is the solid constituent of water No. 1, is soluble in water only when that water also contains carbonic acid gas in solution; on boiling the water, the gas is driven off and the carbonate of lime is no longer soluble.

Baron Bunsen determined the solubility of carbonic acid gas in water as 1.7967 volumes at 32°, and only .9014 volumes at 68°, and, whatever the pressure, the dissolved volume of gas remains constant. Hence, the weight of gas absorbed is proportional to the pressure. The amount dissolved decreases with rise in temperature, until at boilingpoint none is left.

Of the usual impurities in feed-waters, 100 parts of cold water dissolve .0036 of carbonate of lime and as much as .23 of lime sulphate, or gypsum.

At boiling-heat no carbonate remains, but there is still left .21 of the sulphate.

Carbonate of magnesia to the extent of .02 is dissolved cold. This disappears in boiling.

Sulphate of soda is soluble to the extent of 5.02 of the anhydrous salt at 32°, and as much as 50.65 parts at 90°, though 42.65 parts only are soluble at or near boiling

point, showing a decrease beyond a certain temperature.

Sulphate of magnesia, from having a solubility of 24.7 at 32°, attains, at 222°, as high a solubility as 132.5.

As in the case of carbonate of lime, the presence of carbonic acid is necessary to keep in solution the carbonate of magnesia, a similar salt. As water can be freed from carbonates by boiling, carbonate waters are termed "temporarily" hard, to distinguish them from "permanently" hard, or sulphate, waters.

A carbonate water quickly forms scale, because the whole of its dissolved lime precipitates at once when boiling-temperature is reached, but from this it must not be inferred that sulphate waters do not form scale. They do make a scale, and of a worse nature than a carbonate scale, as will now be shown: After some hours the water in a boiler has changed entirely several times and what is then in the boiler is saturated with all the sulphate left behind. It can then dissolve no further quantity, and scale is formed from that time. Further, when the circulation in a boiler is not rapid and the steam-raising surfaces are near the water-surface, steam is actually formed upon the plates, and, as a molecule of water goes off as steam, it leaves the minutest particle of sulphate right upon the plate ready for instant adhesion, and the scale so formed is very tough and tenacious, more so than a carbonate scale.

The great variety observed in the hardness of scale depends upon the admixture of other substances in the water.

A little clay would probably soften a scale by the particles of clay becoming intermixed with those of the lime salt, and the strength of the scale would be decreased.

Carbonate of lime has the chemical symbol CaO, CO2 which shows that it is a compound of lime and carbonic acid. As it exists in water it may be written CaO, CO2+ CO2, the quantity after the sign of addition being the gas which keeps it in solution. If, therefore, to water containing lime carbonate in solution we add plain lime, CaO, which is the oxide of the metal calcium, we cause a peculiar action to take place. The lime absorbs the free gas and the following equation shows what occurs :

(CaO, CO,+CO,)+CaO=2(C0,(0,). We have simply absorbed the gas and made more carbonate, which, with the old carbonate already present, will all precipi

tate. If this process is carried on inside a boiler, the quantity of mud formed will be large, and after all is done in the way of blowing out, we can never prevent some scale from forming.

In boilers of the Lancashire type it is usual for much mud to deposit and harden at the back end of the boiler-bottom; we have seen it there from 6 to 8 inches thick.

The

If time and space allow, the operation can be conducted outside the boiler and the water cleared by slow settlement and filtration, in which case no scale will be formed in the boiler. In the March number a form of apparatus for this purpose will be illustrated. Another plan has been suggested: the addition of sal ammoniac. Sal ammoniac has the formula NHCl, and the reaction with the lime carbonate is as follows: CaO,CO,+2(NH,Cl)=CCl, - (NH,),CO, highly soluble calcium chloride and volatile carbonate of ammonium being formed. lime chloride can be blown out and the ammonium carbonate goes off with the steam. The danger of this is stated to be a tendency to the formation of hydrochloric acid if the salt is added in excess, the decomposition of NHCl giving ammonia, NH, and hydrochloric acid, HCl. The former injures brass or copper and the latter injures the boiler-plates, so that if this method be tried it would be advisable on the last count to make frequent tests of the water drawn from the boiler with litmus paper, which for the water to be safe must be blued by it. If reddened, to correct the evident acidity, we add soda, Na,O, when the action which occurs is Na2O + 2HCI 2NaCl + H2O, or simply common salt and water, which would in time accumulate and require blowing out. In the face of the risk, this method is not to be specially recommended.

The most common present-day practice is the use of soda, NaO2, either in its anhydrous form or as its caustic hydrate NaOH. The effect of this when put into a carbonate water is as follows: CaO, CO2 CO2 + Na,O=CaO, CO2 + Na2CO3.

The lime falls on the subtraction of the free gas, and carbonate of soda is formed; the latter being very soluble, may be neglected for a long time. This process still leaves the sediment in the boiler to be blown out, and is necessarily accompanied by a waste of hot water.

Turning now to sulphate waters, we have for lime sulphate the chemical formula CaSO4, and it is found that if to water containing gypsum we add carbonate of

soda, or NaCO3, the following reaction takes place: CaSO, + Na2CO2 = CaCO3 + Na2SO4. Of these the first is our old friend, carbonate of lime, which precipitates, and the next is sulphate of soda, which is very soluble and harmless, and may be long neglected.

We notice here, then, a curious fact. When we use plain soda for dealing with lime carbonate, we get as one product carbonate of soda, which is just what is needed for dealing with the lime sulphate. This was first pointed out by Dr. Smith, who argued correctly that in water which contains both carbonate and sulphate there is a double decomposition effected, the soda added doing duty first upon the carbonate, and then, when itself turned to carbonate, proceeding to act on the sulphate, thus turning all the dissolved lime salts to mud and itself becoming sodium sulphate. Of course it is necessary to know, in making use of this double reaction, what the ratio is of carbonate to sulphate in a mixed water. It is not necessary to go deeper into the ratio of the combining weights of lime and soda salts, it being sufficient for our present purpose to know that 100 parts by weight of lime carbonate will call for 80 parts of caustic soda, 2(NaHO), to neutralize its suspension and precipitate it, or 62 parts of the anhydrous oxide Na2O.

Again, 100 parts of lime sulphate call for 78 parts of carbonate of soda to precipitate the sulphate as carbonate. Now, 78 parts of carbonate of soda is equal to 45.6 parts of anhydrous caustic soda and to 59 parts of true caustic, 2( NaHO).

The ratio of the caustic soda equivalents in the two cases of lime carbonate and sulphate is 1.36 1.00 or 1: 0.734, that is to say, practically, as 8 to 6. In other words, if a water contains 6 grains of carbonate of lime, the precipitation of this by caustic soda will produce as much carbonate of soda as will then convert 8 grains of lime sulphate into carbonate.

The rules to be followed for different ratios are:

1. Carbonate waters: For each 100 grains of the carbonate of lime add 80 grains of caustic soda.

2. Sulphate waters: For each 100 grains of sulphate of lime add 78 grains of carbonate of soda.

3. Mixture of both lime carbonate and sulphate: For the carbonate add 80 grains of caustic soda per 100 grains, as in rule 1. For the sulphate, subtract from the amount of sulphate per 1,000 gallons, 1.36 times the

weight of lime carbonate. If there is any remainder, add 78 grains of carbonate of soda for each 100 grains, as in rule 2. Thus, if the sulphate does not exceed in quantity 8 grains for every 6 grains of carbonate, it may be neglected, for the caustic soda will be sufficient for this when it has done its part on the lime carbonate. In speaking of caustic soda we refer to the substance known as sodium hydroxide (NaHO), not to the dry powder formed by oxidizing sodium, which is rarely seen commercially.

The commercial caustic is originally made from carbonate by boiling with quicklime, settling, and evaporating the clear solution, and must not be confounded with soda ash, which is, or ought to be, carbonate of soda. Commercial soda ash often contains as much as 50% of impurities, such as the sulphate, sulphite, and chloride of soda.

Before considering how to prevent scale, it is perhaps as well to state that no amount of treatment with soda will remove deposited lime from the inside of a boiler. What soda can do is to facilitate the separating of carbonate of lime and to change sulphate into carbonate.

Before going further, the third class of water calls for notice, namely, acid water. This is not found to accompany carbonate water, but may accompany water containing sulphates. The reason for this is simple. If acid water is mixed with chalk, or carbonate, water, the acid acts upon the carbonate and converts it into sulphate or chloride, until either the acid is neutralized or the carbonate is wholly converted.

The causes of acidity in feed-water are various and sometimes unexpected.

In one case met with by the writer, the explosion of a steam-boiler could be traced to the unexpected acidity of feed-water. The owners of the boiler-new owners-were men who were working several other boilers with the same stream of water and used little or no soda. They treated their newly acquired boiler in the same manner, and it exploded in a few months from rapid internal corrosion. Investigation by the writer and others revealed the fact that the exploded boiler drew water from a waterwheel by-wash, while the uninjured boilers drew from the same stream 200 yards lower down where the by-wash and main body of the stream had become mixed by passing over a fall. Further search revealed a drain discharging "spent acid" from an electroplating works into the by-wash at the same side as the feed-pipe inlet of the exploded