Electricity - voltage, current and resistance

In order to achieve effective electrical stunning, it is helpful to understand the basic principles of electricity.

Ohm’s Law defines the relationship between voltage, resistance and current and states that the current is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit. Therefore, to increase the amount of current flowing through a circuit, the voltage must be increased, or the resistance decreased.

                Ohm’s Law:                        Current (I)  =  Voltage (V)  ÷  Resistance (R)

Voltage is the electromotive force (emf) or electrical pressure that forces the flow of current and is measured in Volts (V). Voltage may also be referred to as the electric potential difference between electrodes. It is necessary to maintain a voltage that is sufficient to produce a current strong enough to ensure that every bird is stunned.

Current (I) is the rate of flow of electric charge through a conductive object and is measured in Amperes (A). The current is the most important parameter in terms of ensuring effective stunning; hence why recommended electrical parameters focus on the current and not the voltage. For example, voltage may vary across different circuits with the same current.

Electrical resistance (R) is a measure of an object’s capacity to impede the flow of current and is measured in Ohms (Ω). Resistance may also be described as impedance, particularly when referring to an object’s resistance to alternating currents. The overall resistance of an object depends on several properties including the length, cross-sectional area and the resistivity of the material that forms the object. The resistance of an object is proportional to its length and inversely proportional to its cross-sectional area. Different materials have different resistances; metals are strong electrical conductors with a low resistance, whereas ceramics, plastics or glass do not conduct electricity well and therefore have a high resistance and are classed as insulators. Whilst it is possible to manufacture the electrodes and the shackles from materials with a relatively high conductance and low resistance, it is not possible to drastically alter the biological resistivity of the birds, although the resistance of living tissue can be reduced by increasing the voltage applied.

An animal is formed of various tissues including skin, muscle and bone which vary in their resistance to electricity. The arrangement of these tissues in the body ultimately determines the path along which the current flows. With time, a voltage progressively overcomes (to a degree) the resistance of the tissue(s) it is passing through, providing a higher current to the tissue. However, electricity is likely to flow along the path of least resistance within an object. Therefore, an applied current is more likely to travel through the lower-resistivity tissues of skeletal (breast) muscle and cardiac muscle than through the more resistive skull bone. It is possible that the brain may only receive a very small proportion of the total current applied to the body, but this may depend on whether a bird’s eye(s) are in physical contact with the electrified water (ie submerged) or the electrified wet plate (read the section ‘Maintaining an uninterrupted electrical circuit and optimising current flow’). Therefore, it is absolutely critical that the minimum recommended currents are delivered, to increase the likelihood that enough current actually penetrates the skull, enters the brain and triggers unconsciousness.

An individual bird’s resistance is highly variable relative to other birds of the same type, as well as between strains, breeds and species (Table 2). Resistance may depend on factors such as age, size (but not necessarily live weight), sex, feather coverage, thickness of the skin and leg scales (degree of keratinization), whether an animal’s skin and/or plumage is wet, muscle and fat composition of the torso, an animal’s state of hydration and the thickness and density of the skull and tarsometatarsal (shank) bones. For example, it was suggested that the greater amount of abdominal body fat, the lower moisture content of body tissues and the thinner legs are the reason why female broiler chickens have a greater electrical resistance than males, despite being almost the same age and weighing less. Females therefore require higher voltages than males, in order to produce the same current amplitude necessary for effective stunning. Similarly, the fat and moisture content of female turkeys and the diameter and surface properties of their legs was suggested as a reason why they have a greater resistance compared to male turkeys.

Table 2. Electrical resistances of poultry.  ♂ = male, ♀ = female. The resistances are based on a 50 Hz sine AC. This table is a guide to the approximate resistances of poultry; because resistance varies with many factors, these values are not guaranteed. Note: mention of foie gras does not imply the HSA agrees with this practice.

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