Excerpts Pertaining to Henry's Galvanic Battery
Henry C. Cameron Notes. Feb 11 th lecture.
“ The simplest form of a Galvanic battery consists of a tumbler containing an acid, into which are plunged two metals for which the acid has an attractions. The acid disolving slightly the one (or rather the oxide of the one, the pure metal being unattacked) recieves a portion from it leaving it negative, and transmits it to the other making it positive. The direction of the current is from the metal acted upon through the acid to the other metal, and thence back again to the first metal when there is a connection, a thin platinum wire placed in the connection upon the passage of the current becomes red hot. The only galvanometer at first known was the frog. Sir H. Davy had a room in which he kept his frogs and killed them as he used the. The room is still shown. If a current of galvanic electricity be sent above a magnetic needle it causes it to turn at right angles and this fact furnishes a galvanometer. A wire is frequently coiled around a needle freely supported & by this instrument we detect the slightest quantity of galvanism. We may use also a vertical needle. Any two metals may be used which have a different affinity for oxygen. The one having the greater affinity becoming minus and the other plus. Thus when zinc and iron are used, the former becomes minus, the latter plus. When we use iron and copper, the iron is minus and the copper plus and the copper again minus when employed in connection with platinum. The action in each case is increased by a decrease of the distance of the plates, though care should be always taken that the plates do not touch, as the action would then go on silently. A simple form of the single battery is to wrap a sheet of zinc in paper, and place around this the copper. The paper prevents contact and yet allows the passage of the acid and electricity, and we thus get a vast amount of electricity the form of the copper also favoring this. A compound battery consists of a number of single batteries arranged so that the positive plate in one cup is connected with the negative plate in the next &c. Wires are attached to both extremities which being brought together complete the circuit. In this case the first positive plate receives a certain quantity of electricity from the negative plate through the acid, which is transmits to the next plate (negative). The second positive plate receives from the second negative plate not only the quantity arising from the solution of the acid but also that received from the first plate and so on to a hundred plates which will have 100A. By thus increasing the number and surface of the plates, we get a powerful battery. Volta in 1798 discovered the pile (of which the battery is only a modification. This consists of plates of zinc resting upon plates of copper with flannel soaked in acid between each pair. This is objectionable on account of the acid being pressed out by the weight at the bottom. Then the action was referred to mere contact. Cruickshanks altered this into the Galvanic trough which is only the pile upon its side. The plates of zinc and copper are soldered together and the acid is between the pairs. The principle of this has been explained above. The effect is increased by passing the discharge through a coil of wire. Explanation of this hereafter.
Dr. Hare of Philadelphia has greatly directed his attention to the improvement and perfection of apparatus and has made many valuable inventions &c expeciallin regards to Galvanic Batteries. For one of them, the plates were so fixed that tey could all be removed ad libitum. This he called the Deflagrator and the cause of its invention was that although the battery acted powerfully at first yet it graduallbecame weaker and finallyalmost ceased to act. The cause of this was unknown until recently. The cause of the deterioration of the pile or battery are (1) the formation of a sulphate of zinc which being diffused throughout the solution settles (in the form of metallic zinc) up on the copper; a counter current of electricity being thus produced which in some degree neutralizes the old one: (2) Local action. The zinc of commerce is very impure containing iron &c: hence [???] of the electricity generated from the zinc is neutralized in this manner. (3) Hydrogen is formed, which being a nonconductor and adhering to the copper, infilms it, thus preventing the speedy passage of the current &c. A remedy for the sulphate of zinc is to put in a partition ofpaper through which the acid passes readily but it with much difficulty.
Galvanic action is explained by means of a chemical theory of which Wollarton (sp?) is the author. Davy Becquiler Faraday and Henry have greatly improved it. The atoms of different bodies are supposed to exist in different electrical states e.g. oxygen is naturally negative and hydrogen positive. The two being brought together, attract, unite and form a drop of water. Pure metal is never acted upon by acid; it must previously be oxidized; this oxide is dissolved off by the acid; this continually presenting anew surface for action &c. The oxide being fromed by the combination of the oxygen in the water with the zinc, the hudrogen as it were imperceptibly escaping at the copper. The zinc being positive causes the oxygen or negative end of an atom of water, by induction, to assume aprximate position & hence we have a row of atoms, disposed as in the first figure opposite. Now the wires being united, the zinc becomes so strongly positive that by induction it causes the atom of oxygen next to it to become more intensely negative so that it leave the atom of hydrogen, unites with one of the zinc forming oxide of zinc &c. The atom of hydrogen thus liberated unites with the oxygen of the next atom of water, and so across to the copper plate where the last atom of hydrogen having no oxygen with which to unite is disengaged & ascends &c. The atoms being free to move arrange themselves as before and the same action takes place. A new row may also presented in succession. If the atoms be not allowed to move inter se (?) e.g. by freezing the water, the above effects will not take place. The figures opposite show the appearances before and after action.
There have been many improvements in the battery during a few years past. (1) A porous membrane is introduced between the plates through which the acid acts, but which prevents the sulphate of zinc from passing over to the copper. (2) By employing different liquids on the different sides of the membrane. The disengaged hydrogen being a non-conductor, infilms the copper plate, thus impeding its action. We pour nitric acid along the surface of the copper plate, a portion of whose oxygen unites with the nascent hydrogen forming water, and itself becomes reduced to nitrous acid. The infilming is thus prevented and a new current is obtained which as added to the previous current from plate to plate. (3) We prevent the local action from interfering by using pure or amalgamated zinc. When coated with mercury, it resists the action of the acid but still produces a current of electricity. We may also use a plate of platinized copper between 2 of zinc. Daniell's constant battery consists of a piece of pure or amalgamated zinc in an envelope of paper containing sulphate of copper (or sulphuric acid, NOTE: it looks like sulphuric acid was written over sulphate of copper) the whole being in a cylindrical vessel composing the copper plate, and containing sulphate of copper/sulphuric acid (again, one written over the other). The sulphate of copper inparts its oxygen to the nascent hydrogen, forming water, and a quantity of pure copper is thus being continually deposited, thereby increasing the thickness of the copper plate. It is thus rendered constant.
Grave's battery which is by far the most powerful of any in use consists of sulphuric acid on the one side and nitric acid upon the other. The plate of zinc is placed in the former and the plate of copper (for which in the more powerful ones, platinum is substituted) is inserted in the latter &c. The copper would be acted upon by the nitric acid, and a counter current thus induced, which would materially interfere with the action. Hence we use platinum. The theory of Ohm in regard to the working power of a battery has supplied a desideratum for which the scientific world has sought for a long time. It is that the effective or working power of the battery is directly as the electromotive power and inversely as the resistance or A =n*E/(n*R+r), in which E = electromotive power, R = resistance of acid, r = resistance of the wire, and n = number of plates. By this we can calculate the power of any battery. (1) The effects of galvanism at rest. Are the Electrical Effects (2) Its effects in motion are Physiological, Mechanical, Magnetic, Chemical, and Calorific. &c &c. There is also Galvanic Induction. The electromotive force increases with number of plates. The Electrical Effects are best shown by the pile of De Luc. This is formed by pasting a coating of tin foil upon one side of a sheet of paper, and black oxide of manganese upon the other, and the dry. Cutting up with a gun punch and laying these circular pieces one upon the other so as to form a pile, the same side of each piece being always up &c. These are placed in a glass tube and sealed up. The moisture of the paste in the paper serves for acid, the manganese for zinc. One end is of course positive and the other negative &c. “
Feb 13th lecture.
“ Experiments in Galvanism etc.
Daniell's constant battery was exhibited in action this morning. It consisted of 22 cylindrical copper jars, so disposed as to form what the French call couronne des tasses1 (french for “crown of glasses”), each containing a plate of amalgamated zinc, separated however by the partition of paper, sulphate of copper was placed in each cell to take up the hydrogen as soon as generated. The quantity of galvanism depends upon the size of the plates, the intensity upon their number. This battery gave sparks and affected the electrometer, thus showing the identity of the galvanic & the electric fluid. Twelve cups of Groves battery are equal in action to the large batteries in the Philosophical Hall which consists of 300 plates upon Dr. Hoare's plan for sudden [??].
Power is matter in an unstable for. Some have though that the brain was a galvanic battery, and that when we exercise our volition in regard to the motion of any part of our body a stream of electricity passes along the nerve caused the muscle to contract &c. But this has not been probed. The researches of Matteuci (sp?) a distinguished Italian have tended to show that the contractions of the muscle are due to the current of electricity existing in the muscle itself and not conducted thither by the nerve from the brain. . .
If we plunge the wire of the batery into water it is decomposed into its elements hydrogen and oxygen, the former going to negative and the latter positive. If we place an inverted glass vessel over each pole so as to receive the gasses when generated, we shall find that there are two parts of hydrogen to one of oxygen. The gasses may afterwards be burnt &c. No action takes place between the poles, (at least microscopic observation reveals none) but over each pole. If the galvanic fluid be sent through a mixture of hydrogen & oxygen it produces and explosion causing them to combine and recompose into water.”
Theory of Henry's Galvanic Battery
Joseph Henry's battery was one of the earliest and simplest types of batteries. It consisted of a copper and a zinc electrode immersed in a solution of dilute sulfuric acid and connected by an external conductor.
To understand the workings of Henry's battery in the language of modern chemistry, it is easiest to first start by understanding the workings of a similar battery, the Daniell cell. An ideal Daniell cell consists of a copper electrode immersed in a 1M solution of copper sulfate and a zinc electrode immersed in a 1M solution of zinc sulfate. The two solutions are connected by a salt bridge or a porous membrane, which allows the flow of positive ions but prevents the mixing of the solutions.
Electrochemical cells like the Daniell cell work due to the occurrence of a specific type of chemical reaction, an oxidation/reduction or “redox” reaction. A redox reaction occurs when one metal gives up electrons, which is known as oxidation, and another metal simultaneously gains those electrons, which is reduction. The site where reduction takes place is called the cathode, and the site where oxidation takes place is known as the anode. To write these reactions using chemical terminology, the oxidation of some given metal M is written as:
M → M n+ + ne -
In the same way, the reduction of a given metal is written as:
M n+ + ne - →M
In this case, n is the number of electrons associated with the reaction. For example, the oxidation of zinc involves two electrons, so that reaction would be written as:
Zn → Zn 2+ + 2e -
On the other hand, the reduction of hydrogen only involves one electron, so that reaction would be:
H + + e - →H
It is very important to note that it is impossible to have oxidation without reduction, and vice versa. Whenever something is being oxidized, something else must be reduced. Each of these reactions is termed a half-reaction, and the total electrochemical reaction is simple a sum of two half reactions. In a Daniell cell, the pertinent reactions are the reduction of copper and the oxidation of zinc. The copper reduction reaction is:
Cu 2+ + 2e - →Cu
Combining that with the zinc oxidation reaction above, the total reaction for the Daniell cell is:
Cu 2+ + Zn →Cu + Zn 2+
In order to understand how this reaction forms a battery, one additional idea of central importance is required, the concept of the electrode potential. The electrode potential is the electric potential (measured in volts) associated with each redox reaction. This potential arises due to the fact that not all metals have the same electronegativity, which is a measure of how easily an element can be ionized. Electronegativity is commonly measured on a dimensionless scale from .7 to 3.3, and a more involved explanation can be found at the end of this section. Electronegativity can also be thought of as a measure of how strongly a metal wants to hold onto its electrons. If two metals with differing electronegativities are brought together, the metal with the lower electronegativity will tend to give up its electrons to the other metal. It is this tendency that causes the voltage difference, and hence the flow of electrons. In this case, the voltage associated with the reduction of copper is .340 volts, and the voltage associated with the oxidation of zinc is -.763 volts. The total voltage difference between the two cells is simple the difference between these two voltages: .340 - -.763 = 1.103V
It is difficult to determine the voltage associated with a given redox reaction, since the half-reactions cannot occur independently and only the difference in the voltages can be directly measured. To solve this problem, the reduction/oxidation of hydrogen was defined as having an electrode potential of 0V, and all half-cell potentials are the difference between the oxidation/reduction of the element in question and the oxidation/reduction of hydrogen - positive values mean that the element causes hydrogen to be oxidized and negative values mean it causes hydrogen to be reduced. All these voltages are experimentally determined, and many tables for the different elements are available.
Additionally, the potentials are defined for a electrode of pure metal immersed in a 1M solution of its ions at 25 C, which is important because both temperature and ion concentration have an effect on the voltage produced. This effect is described by the Nernst equation, which gives the change in voltage from the standard potential caused by changes in concentration and temperature.
Since the copper has a higher electronegativity (1.90) than zinc (1.65), the electrons tend to leave the zinc atoms and travel to the copper, creating a current through the connecting wire. Cu 2+ ions pick up these electrons and become Cu atoms that are deposited on the copper electrode. The Zn atoms at the zinc electrode become Zn 2+ ions that go into the solution, leading to the eventual total corrosion of the zinc electrode. To prevent a charge imbalance, at the same time positive H + ions will flow through the porous membrane from the zinc sulphate to the copper sulfate, and SO4 - ions will flow in the opposite direction. The flow of electrons through the wire connecting the zinc and copper electrode is the current that gives the battery its effectiveness.
The batteries used by Joseph differed slightly but significantly from the one described above. They still used zinc and copper electrodes, but the electrodes were not separated in two containers nor were the immersed in solutions of their ions but simply placed together in a solution of dilute sulfuric acid (just how dilute is currently a matter of some uncertainty). This means that the reaction changes fundamentally from the one in the Daniell cell because copper is no longer the reducing agent as there are no free copper ions in the solution to be reduced. Instead, hydrogen becomes the reducing agent, and so the reaction becomes
2H + + Zn →H2 + Zn 2+
When the copper and zinc are joined together by an external conduction hydrogen bubbles noticeably form on the surface of the copper, indicating that this is the reaction taking place.
This lowers the theoretical voltage of the battery - since hydrogen is defined as 0V the theoretical voltage becomes only .763 volts. However, for an acid concentration of .8M, this becomes approximately .87 volts, which was measured experimentally.
The internal resistance of Henry's batteries is hard to know for sure. Replicas of his batteries have had measured resistances of 3-5 ohms, which is quite high (most modern batteries have an internal resistance of less than .1 ohm). However, it is unclear what molarity his acid was, and the higher the acid concentration the lower the internal resistance, to a point. Larger surface area also reduces internal resistance. However, his experiments with the Albany magnet and others seem to suggest his batteries had quite low internal resistances. One possible explanation is that his acids were, as opposed to being more concentrated, actually far more dilute than .8M, which is the concentration at which some preliminary measurements were made. When immersed in .8M acid, zinc corrodes, giving off hydrogen gas that bubbles on the surface in a separate reaction than the one that creates the battery. This reaction continues regardless of whether the zinc is joined to the copper electrode or not, and is more or less vigorous depending on the concentration of the acid. The large number of bubbles forming on the zinc electrode effectively removes almost all of the surface area, which would cause the internal resistance to increase dramatically. If the acid concentration were low enough that this corrosion did not occur, then much more of the surface area of the zinc would be exposed, and the internal resistance would then decrease proportionally. Further study in this area is required.