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Weiss therefore discovered the mathematical relationship that links the pulse duration with the amount of
electricity required to produce the stimulation.
Understandably, he called this relationship the “fundamental formula”:
𝑄 = 𝑞 + 𝑖𝑡
𝑄
= the amount of current required to reach the threshold. This is also the quantity of
electrical charges provided by the stimulation current, as the
𝑄
value is given by the
product (
𝐼 𝑥 𝑡
) of the stimulation current intensity multiplied
by its application time.
𝑡
= length of time that the current is applied, which is known as the pulse duration.
𝑖
= a coefficient determined by experiment, with the same
quantity as an electric current (intensity).
𝑞
= a coefficient determined by experiment, with the same dimensions as a quantity
of electrical charges;
𝑞
corresponds to the intersection of the straight line with the
y-axis and may be calculated as the
𝑄
value when
𝑡
is equal to zero.
Lapicque, an electrophysiologist who is more widely known than Weiss, did not actually discover a new
law of electrostimulation but he performed a number of experiments which confirmed the fundamental
formula. He defined it differently to mathematically deduce coefficients called the rheobase and chronaxy,
which he gave physiological meaning.
Lapicque developed the “fundamental formula” as follows:
Q = q + it
or
Q = It
I
: stimulation current intensity
t
: pulse duration
therefore
It = q + it
by dividing the two by
𝑡
, Lapicque obtained
𝐼= 𝑞 /𝑡 + 𝑖
which is the relationship between the intensity of the current and the time period in
which it must be applied to achieve stimulation (Fig. 3).