A. VOLTAGE SURVEYS

The number and character of the potential readings required depend on the information desired. As previously pointed out, the readings depend on the thoroughness of the investigation to be made, and it is to be understood that many of the measurements described below would often not be necessary, and in general would be taken only during the course of a complete electrolysis survey. Voltage surveys are here divided into two main classes: (a) Voltage measurements between two points on the same structure, and (b) Measurement of the potential difference between structures.

1. Measurement of Maximum Potential Drop Along Railway Structures.

(a) Importance of Maximum Potential Drop Measurements. Such measurements, when interpreted in the light of other conditions to be discussed later, afford a valuable index to electrolysis conditions generally. It is, further, one of the easiest quantities to determine in an electrolysis survey if the use of telephone lines can be secured. These measurements show in general whether the railway system is properly maintained and what lines or sections are most in need of repair and rebonding. When taken in conjunction with the load data, they may be used for an approximate calculation of power losses in the railway return and when studied with due regard to the character and location of railway lines and supply stations, together with the distribution of load, they afford a valuable index as to the need of, or the modification of the track feeder system. It is therefore desirable, as a rule, to take a good many of these measurements as a part of any complete electrolysis survey.

(b) Procedure in Making Maximum Potential Drop Measurements. The first step in making measurements of this kind in a city is to determine the location of points between which potentials are to be observed. These usually comprise points on the track most remote from the power supply station as the points of highest potential, and the points on the track nearest the power station as the point of lowest potential. In some cases, however, especially where insulated track feeders are used, the point of lowest potential may be at the point of connection of one of the insulated feeders which may be at a considerable distance from the power station. It is desirable, as a rule, to measure the difference of potential between the points of connection to the tracks of all of the insulated track feeders in order that the points of lowest potential may be determined. It is desirable, as a rule, also to select points intermediate between the points of highest and lowest potential so that the distribution of the potential drop may be determined, as this will give a valuable insight into the location of bad stretches of track and of concentration of return current in the tracks.

Reference should be made to a positive feeder map from which a list of power stations and their approximate feeding distances can be determined. Special lines to these points may be run or spare wires may be borrowed or leased from the telephone company. In the latter case,

the continuous cooperation of the telephone company is required. Having a list of points to be reached, the telephone representatives can prepare a table showing the terminal boxes and numbers of spare pairs, including trunk lines which are necessary to make a complete circuit between one of the telephone central offices, or other suitable central point where the measuring instruments are to be placed and the points to which measurements are to be made. All measurements can then be made between the point of lowest potential and all other points selected, and between any two points as desired. Temporary circuits are necessary when no spare telephone conductors are available, in which case working conductors may sometimes be used for short periods. In most cases, it is desirable to make permanent connections to the track for maximum voltage drop measurements, but where, for any reason, permanent or semi-permanent connections to the track cannot be made, temporary connections become necessary and the installation of these temporary connections will require considerable time and expense for labor.

It will be found most convenient to bring all the lines from the various points on the track network to a large board on which is mounted a map of the railway system, each wire being fastened to a binding post located at a point on the map corresponding to the point on the track from which the wire comes. Once the correct connection of wires has been verified, one can readily connect the voltmeter to wires leading to any points in the city without possibility of error.

While making track voltage measurements, and, in fact, all other measurements, it is desirable to arrange to have the test data worked up and tabulated so that it can be carefully studied as the work progresses This is important because, as pointed out above, the tests to be made during the course of the survey often depend in large measure on the results of preliminary measurements so that by making a study of the preliminary data while the work is in progress, it is often possible to modify original plans in such a way as to greatly increase the value of the test data.

In making electrolysis measurements, it is desirable to take readings at each point over as long a period as practicable. Owing to the great variability of railway loads, it is important to have the readings cover at least one complete cycle of the load, and often several complete cycles are desirable. It is quite common practice to take such readings over a period of one hour, but in some cases, especially on interurban lines and others where the schedule is very infrequent, still longer periods may be necessary. Even where readings are taken over one hour it will generally be necessary for comparative purposes to reduce these readings to an equivalent twenty-four-hour value, and in some cases also corrections have to be made for seasonal changes of the load. This matter will be treated at some length under the discussion of the interpretation of electrolysis survey data.

2. Potential Gradient Measurements.

(a) Scope of Term. Under the head of potential gradients will be included all potential measurements between different points on the track

or between different points in the earth spaced materially less than the extreme feeding distances within the powerhouse areas.

(b) Measurement of Potential Gradients in Tracks. Potential gradient measurements are usually made on the railway tracks, but at times also on pipe systems or even in the earth. The procedure will vary considerably because of the variability of the distances over which measurements are to be made. If the spans are long, telephone wires are the most convenient and the measurements are made in the same way as track voltage measurements described above. Where the distances are relatively short, however, as for example, a few hundred feet or less, a temporary wire between the two points of measurement will usually be most convenient. Connections to the system on which measurements are made will depend on whether the tests are being made between points on the tracks or on the pipe system or between points in the earth. For measurements between points on the tracks or on the pipe system or other metallic structure metallic terminals may be firmly held against the rail or pipe or a wire may be swedged in a slot sawed in the pipe or rail under test. It is sometimes desirable to make potential measurements directly between two points in the earth, though the most common practice has been to take them on the track network. Special situations may arise where potential measurements between various points in the earth are more valuable than those taken on the underground structures. For example, the presence and direction of large transverse currents in the vicinity of important mains can be determined. Buried pipe lines or other conductors at uncertain locations which are discharging current into the earth may be located approximately by earth gradient measurements, there being a reversal or abrupt change in the gradient when the conductor is crossed.

In making earth gradient measurements between points relatively close together, it is essential that a pair of non-polarizable electrodes be used if a high degree of accuracy is to be attained. Such electrodes are now in process of development at the Bureau of Standards.

The periods over which gradient measurements should be made and the procedure in working up the data during the progress of the survey are governed by the same considerations as discussed above in the treatment of the track voltage measurements.

3. Measurement of Potential Differences.

(a) Purpose of Measurement of Potential Differences. Measurement of potential differences are beyond question the measurements most frequently made in connection with electrolysis tests and when their limitations are properly taken into account, they afford a valuable index to electrolysis conditions. It should be emphasized, however, that they are chiefly of qualitative significance, being valuable for indicating the region in which more or less damage to pipes may be in progress, but not giving any definite information as to the rate at which injury to pipes may be progressing. This is due to the fact that the resistivity of the earth and railway roadbed varies with local conditions, that is, a given potential difference that would be practically safe under some conditions of soil

resistance would be extremely hazardous in other locations. If this factor is properly taken into account, potential difference measurements may be of considerable value in determining electrolysis conditions.

(b) Procedure in Making Measurements of Potential Differences. Measurements of potential differences between adjacent structures should be made at many points between fire hydrants, lamp posts or gas or water services and tracks, lead cable sheaths and tracks, lead cables and accessible portions of pipe systems, between any two pipe systems that approach closely to each other, and where practicable between cable systems and the earth. In making contacts on fire hydrants and lamp posts, care should be taken to make contact with the pipe itself, rather than the housing. These measurements, between cable systems and earth, if properly taken, afford the most valuable index of electrolysis conditions, but. unfortunately, they are the most difficult to secure and, unless taken by a competent engineer, thoroughly familiar with the possible sources of error involved, they may be worthless or actually misleading. These measurements when taken should be made throughout a large part of the piping or cable networks, including any regions in which there is reason to believe that stray current may be leaving the affected structures for the earth.

Since the structures between which potential difference measurements are made are usually close together, short leads only are required, short lengths of lamp cord or other flexible wire being most commonly used. Either temporary or more or less permanent connections to metallic struc tures may be made, according to whether readings are to be taken over a short or long period and whether they are to be repeated at some future time. When measuring potential differences between pipe or cable systems and the earth, it is important to use an auxiliary earth electrode that is known to give a very small galvanic potential against the metal of the structure under test. For lead cables a piece of lead sheath is entirely satisfactory. In the case of iron pipes the problem is more difficult because of the variability of iron and the possibility of complication due to oxidation of either the pipe under test or the auxiliary iron electrode. When such readings with iron electrodes amount to only a few tenths of a volt they should not be regarded as reliable unless taken over a period including that during which the railway power station is shut down, owing to the possibility of galvanic voltages being of this order of magnitude.

B. CURRENT SURVEYS

1. Scope and Importance of Current Measurements

Under the head of current measurements are included all observations of current flow obtained by ammeter readings, or by a potential drop on a conductor, the resistance of which is approximately known. They include measurements of current flowing from subsurface structures into the earth.

Current measurements on undrained structures made both before and after a change in the railway system or the application of other mitigative measures afford considerable information as to the change in electrolysis conditions. Owing to the great variety of conditions under which it is

at times necessary to measure current, as in copper feeders, rails, pipes, cable sheaths, and even in portions of the soil, the methods of procedure may vary considerably.

2.

Measurement of Currents in Feeders and Rails.

(a) Purpose of Measuring Feeder and Rail Currents. Measurements of current in track feeders and rails are usually made only when it is desired to check the current distribution in a network of tracks. Current measurements on the track will show the points at which additional track feeders are required in order to limit potential gradients in the track as well as the amount of current that must be taken off at each point, and consequently the sizes of feeders required. The same result can be obtained with sufficient accuracy for most practical purposes by the use of a "spot map" on which are shown the average distribution of cars and their corresponding loads. Further, by measuring current in different rails in the track, local bad bonding will be revealed, since unequal distribution of current always indicates relatively high resistance in the rails carrying the lower currents. In fact, some engineers regard the measurement of the relative current in the rails at a number of points as the most reliable way of obtaining in a short time a good idea of the condition of track bonding.

(b) Procedure in Measuring Current in Feeders and Rails. The most accurate method of measuring current in a feeder of rail is, of course, afforded by inserting an ammeter shunt directly in series with the feeder or rail under test. However, in practice it often happens that in the case of negative feeders ammeters or shunts are not provided and can be inserted only with difficulty, and in the case of rails this is impracticable. The most common method, therefore, of measuring current in such structures is to measure the potential drop on a known length of cable or rail and to calculate the current from this potential drop and the resistance of the conductor. Such measurements of current can be made on copper cables with high accuracy and on steel rails the results can usually be relied upon to 10 per cent. or better, which is sufficient for practically all purposes. In making the current calculations it is customary to consider the resistivity of the copper at 10.7 ohms per circular mil-foot, and that of steel rails to be 0.0003 ohm per pound-foot, this latter being equivalent to a resistance of 0.000009 ohm for one foot of rail weighing 100 pounds per yard. In practice it may be expected, however, that the resistance per pound-foot may vary between the values of 0.00027 and 0.00033, or about ten per cent. each way from the mean values here given. Table 5 in the appendix will be found convenient for calculating the current in rails of various weights.

3. Measurement of Currents in Pipes and Cable Sheaths.

(a) Purpose and Importance of Pipe Current Measurements. The measurement of current in pipes and lead cable sheaths is important for a number of reasons. Heavy currents in pipes are often objected to by owners of pipe networks, particularly gas and oil pipes, owing to the fear that trouble may result from ignition of gas or oil due to arcing when