Meaning, in general, ‘comfort’
‘Hot’ and ‘cold’ are sensations for the horse that depend on the rate at which heat energy is transferred from, or to, an object in contact with it. This rate depends on the temperature difference between the surface of the object and the surfaces of the horse’s mouth as well as the thermal diffusivities of both structures. In physics, the horse, being far more massive and warmer than the bit, is called the ‘thermal bath’ and the bit will heat so that it takes up the temperature of this warm ‘bath’. In this case we can consider the thermal diffusivity of the bit material alone and compare between different materials.
In materials with very low diffusivity, such as polymers, the temperature difference at the contact interface becomes very small a short time after first contact. However, the bit will not fully reach a uniform temperature throughout its volume for a long time. This can feel comfortable because of the small temperature difference at the interface and the very slow rate at which heat energy is transferred from the horse to the center of the bit.
Conversely, in materials with very high thermal diffusivity, such as copper, the rate of heat energy transfer is very fast and a high temperature difference between the bit and the horse lasts for the short time it takes for full heat transfer from the horse to the bit. Throughout this the bit will feel ‘cold’ before it rapidly acquires the temperature of the ‘bath’ and will then feel comfortable. The aim is to make this heat transfer time as short as possible.
In between these extremes, variable degrees of discomfort are possible for longer times because the heat transfer rate is intermediate and the large temperature difference and moderate rate of heat transfer maintains the ‘cold’ feeling for longer. Salox comprises copper for high thermal diffusivity, but with necessary strengthening and anti-tarnishing agents. The ideal target would aim to meet a thermal diffusivity level that allows the bit to thermally equilibrate on a time scale comparable to the reaction time of the central nervous system of the horse.
The following table and graph gives our experimental values for thermal diffusivity comparing Salox and a typical ‘standard’ copper alloy (SCA) bit material and gives literature values typical for stainless steel and for a high quality engineering polymer that may also be used for horse bits.
Table 1: Thermal diffusivity values (mm2 s-1) measured (SCA and Salox) or taken from literature sources for various horse bit mouthpiece materials
Figure 1: Equilibrium uniform temperature versus normalised time of materials of identical volume and surface dimensions when placed in contact with a thermal bath at 37 °C. The curves are calculated as inverted Newtonian cooling curves with the diffusivities from Table 1 entered for the exponential rate constant
These results show that Salox and the polymer lie at opposite extremes of the scale where comfort is most likely to be achieved. Other materials lie in between these extremes.